diff --git a/scripts/init-repo-github-defaults.ps1 b/scripts/init-repo-github-defaults.ps1
new file mode 100644
index 0000000..8c5f73d
--- /dev/null
+++ b/scripts/init-repo-github-defaults.ps1
@@ -0,0 +1,93 @@
+<#
+.SYNOPSIS
+    Apply opinionated GitHub repo-level defaults to a target repo via gh CLI.
+
+.DESCRIPTION
+    Idempotent: detect current state, only PATCH when divergent.
+
+    Settings applied:
+        delete_branch_on_merge = true   GitHub auto-deletes head branches
+                                        on PR merge. Eliminates the slow
+                                        drift of stale merged-PR branches
+                                        that this script was written to
+                                        prevent (captured 2026-05-18 after
+                                        a marathon session left 8 orphans).
+
+    Documented as a cross-project pattern in the vault at
+    00_meta/patterns/github-branch-hygiene.md.
+
+.PARAMETER Repo
+    Target repo as owner/name. Defaults to the current repo's origin remote.
+
+.PARAMETER DryRun
+    Show what would change without applying.
+
+.NOTES
+    Requires the gh CLI authenticated (gh auth status).
+    ASCII-only per project policy (PSScriptAnalyzer PSUseBOMForUnicodeEncodedFile).
+#>
+param(
+    [string]$Repo,
+    [switch]$DryRun
+)
+
+Set-StrictMode -Version Latest
+$ErrorActionPreference = 'Stop'
+
+$ghCmd = Get-Command gh -ErrorAction SilentlyContinue
+if (-not $ghCmd) {
+    Write-Error 'gh CLI not found. Install: https://cli.github.com/'
+    exit 3
+}
+
+# --- Resolve repo ---
+if (-not $Repo) {
+    try {
+        $originUrl = (& git remote get-url origin 2>$null) -replace "`r`n|`r|`n", ''
+    } catch {
+        $originUrl = ''
+    }
+    if (-not $originUrl) {
+        Write-Error 'Not in a git repo or no origin remote. Use -Repo <owner/name>.'
+        exit 4
+    }
+    # Strip git@github.com: or https://github.com/ and trailing .git
+    $Repo = $originUrl -replace '^git@github\.com:', '' `
+                       -replace '^https?://github\.com/', '' `
+                       -replace '\.git$', ''
+    if ($Repo -notmatch '^[A-Za-z0-9_.-]+/[A-Za-z0-9_.-]+$') {
+        Write-Error "Could not derive owner/name from origin ($originUrl). Use -Repo."
+        exit 5
+    }
+}
+
+Write-Host "[INFO] Target repo: $Repo"
+
+# --- Current state ---
+try {
+    $currentState = & gh api "/repos/$Repo" --jq '.delete_branch_on_merge' 2>$null
+    $currentState = "$currentState".Trim()
+} catch {
+    $currentState = 'unknown'
+}
+
+Write-Host "[INFO] Current delete_branch_on_merge: $currentState"
+
+if ($currentState -eq 'true') {
+    Write-Host '[OK] Already enabled, nothing to do.'
+    exit 0
+}
+
+# --- Apply ---
+if ($DryRun) {
+    Write-Host "[DRY-RUN] Would PATCH /repos/$Repo with delete_branch_on_merge=true"
+    exit 0
+}
+
+try {
+    & gh api -X PATCH "/repos/$Repo" -f 'delete_branch_on_merge=true' --jq '.delete_branch_on_merge' 2>&1 | Out-Null
+    Write-Host "[OK] delete_branch_on_merge enabled on $Repo"
+} catch {
+    Write-Error "PATCH failed: $_"
+    exit 6
+}
diff --git a/scripts/init-repo-github-defaults.sh b/scripts/init-repo-github-defaults.sh
new file mode 100755
index 0000000..ef6ac9c
--- /dev/null
+++ b/scripts/init-repo-github-defaults.sh
@@ -0,0 +1,97 @@
+#!/bin/bash
+# init-repo-github-defaults.sh
+# Purpose: Apply opinionated GitHub repo-level defaults to a target repo
+#          via `gh api`. Idempotent (detect-current-state, patch only when
+#          a setting diverges).
+#
+# Why: avoids the slow drift of stale merged-PR head branches on origin.
+#      Empirically captured 2026-05-18 after a marathon session left 8
+#      orphan branches because `gh pr merge` was used without
+#      `--delete-branch` and the repo did not have
+#      `delete_branch_on_merge` enabled.
+#
+# Documented as a cross-project pattern in vault:
+#   00_meta/patterns/github-branch-hygiene.md
+#
+# Usage:
+#   ./init-repo-github-defaults.sh [--repo <owner/name>] [--dry-run]
+#
+# If --repo is not given, derives owner/name from the current repo's
+# `origin` remote URL.
+
+set -euo pipefail
+
+usage() {
+    cat <<'EOF'
+Usage: init-repo-github-defaults.sh [--repo <owner/name>] [--dry-run]
+
+  --repo <owner/name>   target repo (default: derived from `origin` remote)
+  --dry-run             show diffs without applying
+  -h, --help            show this help
+
+Settings applied:
+  delete_branch_on_merge = true   GitHub auto-deletes the head branch when
+                                  a PR is merged (squash, rebase, or merge
+                                  commit). Eliminates the orphan branch
+                                  drift this script was written to prevent.
+
+Requires: gh CLI authenticated (`gh auth status`).
+EOF
+}
+
+TARGET_REPO=""
+DRY_RUN=0
+while [[ $# -gt 0 ]]; do
+    case "$1" in
+        --repo) TARGET_REPO="$2"; shift 2 ;;
+        --dry-run) DRY_RUN=1; shift ;;
+        -h|--help) usage; exit 0 ;;
+        *) printf '[ERROR] Unknown argument: %s\n' "$1" >&2; usage >&2; exit 2 ;;
+    esac
+done
+
+if ! command -v gh >/dev/null 2>&1; then
+    printf '[ERROR] gh CLI not found. Install: https://cli.github.com/\n' >&2
+    exit 3
+fi
+
+# --- Resolve repo ---
+if [[ -z "$TARGET_REPO" ]]; then
+    if ! ORIGIN_URL="$(git remote get-url origin 2>/dev/null)"; then
+        printf '[ERROR] Not in a git repo or no `origin` remote. Use --repo <owner/name>.\n' >&2
+        exit 4
+    fi
+    # Handle both SSH (git@github.com:owner/name.git) and HTTPS forms
+    TARGET_REPO=$(printf '%s\n' "$ORIGIN_URL" \
+        | sed -E 's#^git@github\.com:##; s#^https?://github\.com/##; s#\.git$##')
+    if ! [[ "$TARGET_REPO" =~ ^[A-Za-z0-9_.-]+/[A-Za-z0-9_.-]+$ ]]; then
+        printf '[ERROR] Could not derive owner/name from origin (%s). Use --repo.\n' "$ORIGIN_URL" >&2
+        exit 5
+    fi
+fi
+
+printf '[INFO] Target repo: %s\n' "$TARGET_REPO"
+
+# --- Current state ---
+CURRENT_STATE=$(gh api "/repos/$TARGET_REPO" --jq '{delete_branch_on_merge}' 2>/dev/null || echo '{}')
+CURRENT_DELETE_ON_MERGE=$(printf '%s\n' "$CURRENT_STATE" | grep -oE 'true|false' | head -1 || echo 'unknown')
+
+printf '[INFO] Current delete_branch_on_merge: %s\n' "$CURRENT_DELETE_ON_MERGE"
+
+if [[ "$CURRENT_DELETE_ON_MERGE" = "true" ]]; then
+    printf '[OK] Already enabled, nothing to do.\n'
+    exit 0
+fi
+
+# --- Apply ---
+if [[ "$DRY_RUN" -eq 1 ]]; then
+    printf '[DRY-RUN] Would PATCH /repos/%s with delete_branch_on_merge=true\n' "$TARGET_REPO"
+    exit 0
+fi
+
+if gh api -X PATCH "/repos/$TARGET_REPO" -f delete_branch_on_merge=true --jq '{delete_branch_on_merge}' 2>&1; then
+    printf '[OK] delete_branch_on_merge enabled on %s\n' "$TARGET_REPO"
+else
+    printf '[ERROR] PATCH failed (insufficient permissions, or repo archived?)\n' >&2
+    exit 6
+fi
diff --git a/tests/init-repo-github-defaults.bats b/tests/init-repo-github-defaults.bats
new file mode 100644
index 0000000..a5335f6
--- /dev/null
+++ b/tests/init-repo-github-defaults.bats
@@ -0,0 +1,85 @@
+#!/usr/bin/env bats
+# Tests for scripts/init-repo-github-defaults.sh
+
+setup() {
+    export REPO_DIR="$BATS_TEST_DIRNAME/.."
+    export SCRIPT="$REPO_DIR/scripts/init-repo-github-defaults.sh"
+    export PS_SCRIPT="$REPO_DIR/scripts/init-repo-github-defaults.ps1"
+}
+
+@test "init-repo-github-defaults.sh exists and is executable" {
+    [ -x "$SCRIPT" ]
+}
+
+@test "init-repo-github-defaults.sh passes bash syntax" {
+    bash -n "$SCRIPT"
+}
+
+@test "init-repo-github-defaults.sh passes zsh syntax" {
+    zsh -n "$SCRIPT"
+}
+
+@test "init-repo-github-defaults.sh uses set -euo pipefail" {
+    grep -q 'set -euo pipefail' "$SCRIPT"
+}
+
+@test "init-repo-github-defaults.sh PATCHes delete_branch_on_merge=true" {
+    grep -qE 'delete_branch_on_merge=true' "$SCRIPT"
+}
+
+@test "init-repo-github-defaults.sh is idempotent (early-exit on already-enabled)" {
+    grep -q 'Already enabled, nothing to do' "$SCRIPT"
+}
+
+@test "init-repo-github-defaults.sh supports --dry-run" {
+    grep -qE '\-\-dry-run' "$SCRIPT"
+}
+
+@test "init-repo-github-defaults.sh supports --repo override" {
+    grep -qE '\-\-repo' "$SCRIPT"
+}
+
+@test "init-repo-github-defaults.sh --help works" {
+    run "$SCRIPT" --help
+    [ "$status" -eq 0 ]
+    [[ "$output" == *"Usage:"* ]]
+}
+
+@test "init-repo-github-defaults.sh rejects unknown args" {
+    run "$SCRIPT" --bogus
+    [ "$status" -ne 0 ]
+}
+
+@test "init-repo-github-defaults.sh derives owner/name from SSH origin URL" {
+    # Sanity-check the sed pipeline that strips git@github.com: / .git
+    grep -qF 'git@github' "$SCRIPT"
+    grep -qF 'https?://github' "$SCRIPT"
+    grep -qF '.git$' "$SCRIPT"
+}
+
+# --- PowerShell parity ---
+
+@test "init-repo-github-defaults.ps1 exists" {
+    [ -f "$PS_SCRIPT" ]
+}
+
+@test "init-repo-github-defaults.ps1 is ASCII-only (PSScriptAnalyzer rule)" {
+    # No em dashes, arrows, smart quotes, ellipsis -- the recurring CI failure.
+    ! grep -nP '[^\x00-\x7F]' "$PS_SCRIPT"
+}
+
+@test "init-repo-github-defaults.ps1 PATCHes delete_branch_on_merge=true" {
+    grep -qE "delete_branch_on_merge=true" "$PS_SCRIPT"
+}
+
+@test "init-repo-github-defaults.ps1 is idempotent (early-exit on already-enabled)" {
+    grep -q "Already enabled" "$PS_SCRIPT"
+}
+
+@test "init-repo-github-defaults.ps1 supports -DryRun switch" {
+    grep -qE '\[switch\]\$DryRun' "$PS_SCRIPT"
+}
+
+@test "init-repo-github-defaults.ps1 derives owner/name from origin" {
+    grep -qE 'remote get-url origin' "$PS_SCRIPT"
+}