diff --git a/setup-linux.sh b/setup-linux.sh
index a174b58..565340a 100755
--- a/setup-linux.sh
+++ b/setup-linux.sh
@@ -2,6 +2,11 @@
 
 # setup-linux.sh: Install dotfiles and set up environment
 # Usage: ./setup-linux.sh
+#
+# Philosophy: opportunistic configuration. The script attempts to set up
+# every supported tool and warns when something is not available, rather
+# than requiring opt-in flags. Heavy / disk-intensive installs (e.g.
+# future Ollama, Ghostty) may add explicit flags case by case.
 
 set -euo pipefail
 
@@ -424,26 +429,33 @@ else
     log_warning "opencode binary not reachable at $OPENCODE_BINARY after install — agent unavailable"
 fi
 
-# GitHub Copilot CLI (requires gh)
+# GitHub Copilot CLI (detect-and-act: deploy config only when the extension
+# is actually installed; the script does NOT auto-install the extension on
+# Linux — gh is widely used for PR/issue management without Copilot intent.
+# To enable: gh extension install github/gh-copilot, then re-run setup).
+# Verification string updated for AI-013 pointer-style copilot-instructions.md.
 if command -v gh >/dev/null 2>&1; then
-    log_info "Installing GitHub Copilot CLI extension..."
-    gh extension install github/gh-copilot 2>/dev/null || true
-    ensure_directory "$HOME/.copilot"
-    cp -rf "$CURRENT_DIR/ai/copilot/"* "$HOME/.copilot/" 2>/dev/null || true
-    if [ -f "$HOME/.copilot/copilot-instructions.md" ] && grep -q "Engineering Discipline" "$HOME/.copilot/copilot-instructions.md"; then
-        log_success "Copilot instructions deployed successfully (verified)"
+    if gh extension list 2>/dev/null | grep -q "github/gh-copilot"; then
+        log_info "GitHub Copilot CLI extension detected, deploying configuration..."
+        ensure_directory "$HOME/.copilot"
+        cp -rf "$CURRENT_DIR/ai/copilot/"* "$HOME/.copilot/" 2>/dev/null || true
+        if [ -f "$HOME/.copilot/copilot-instructions.md" ] && grep -q 'First, read `AGENTS.md`' "$HOME/.copilot/copilot-instructions.md"; then
+            log_success "copilot-instructions.md deployed successfully (verified pointer to AGENTS.md)"
+        else
+            log_warning "copilot-instructions.md deployment failed verification (expected pointer to AGENTS.md)"
+        fi
+        log_success "GitHub Copilot CLI configured"
+
+        # Aliases
+        log_info "Adding Copilot aliases to .zshrc/.bashrc..."
+        COPILOT_SUGGEST='eval "$(gh copilot alias -- bash)"'
+        [ -f "$HOME/.zshrc" ] && ensure_line_in_file "$HOME/.zshrc" "$COPILOT_SUGGEST"
+        [ -f "$HOME/.bashrc" ] && ensure_line_in_file "$HOME/.bashrc" "$COPILOT_SUGGEST"
     else
-        echo "❌ Error: Copilot instructions deployment failed verification"
+        log_info "GitHub Copilot CLI extension not installed, skipping Copilot config (install with: gh extension install github/gh-copilot)"
     fi
-    log_success "GitHub Copilot CLI configured"
-    
-    # Aliases
-    log_info "Adding Copilot aliases to .zshrc/.bashrc..."
-    COPILOT_SUGGEST='eval "$(gh copilot alias -- bash)"'
-    [ -f "$HOME/.zshrc" ] && ensure_line_in_file "$HOME/.zshrc" "$COPILOT_SUGGEST"
-    [ -f "$HOME/.bashrc" ] && ensure_line_in_file "$HOME/.bashrc" "$COPILOT_SUGGEST"
 else
-    log_warning "GitHub CLI (gh) not found, skipping Copilot installation"
+    log_warning "GitHub CLI (gh) not found, skipping Copilot setup"
 fi
 
 # Register MCP servers (requires Claude Code CLI, Node.js, jq)
diff --git a/setup-windows.ps1 b/setup-windows.ps1
index 562d873..4591dc4 100644
--- a/setup-windows.ps1
+++ b/setup-windows.ps1
@@ -792,32 +792,43 @@ if (Test-Path $ClaudeSettings) {
 
 Write-Info "Setting up GitHub Copilot CLI..."
 
+# Detect-and-act: deploy config only when gh AND the gh-copilot extension
+# are actually installed. The script does NOT auto-install the extension --
+# gh is widely used for PR/issue management on machines that do not want
+# Copilot. To enable: gh extension install github/gh-copilot, then re-run.
+# Verification string updated for AI-013 pointer-style copilot-instructions.md.
 $ghCmd = Get-Command gh -ErrorAction SilentlyContinue
 if ($ghCmd) {
-    Write-Info "Installing GitHub Copilot CLI extension..."
+    $copilotInstalled = $false
     try {
-        & gh extension install github/gh-copilot 2>$null
+        $extensions = & gh extension list 2>$null
+        $copilotInstalled = ($extensions -match 'github/gh-copilot')
     } catch {
-        # Extension may already be installed
+        $copilotInstalled = $false
     }
 
-    $CopilotHome = "$env:USERPROFILE\.copilot"
-    Ensure-Directory $CopilotHome
-
-    $copilotSource = "$DotfilesDir\ai\copilot"
-    if (Test-Path $copilotSource) {
-        Copy-Item "$copilotSource\*" "$CopilotHome\" -Recurse -Force -ErrorAction SilentlyContinue
-        if ((Test-Path "$CopilotHome\copilot-instructions.md") -and
-            (Select-String -Path "$CopilotHome\copilot-instructions.md" -Pattern "Engineering Discipline" -Quiet)) {
-            Write-Success "Copilot instructions deployed successfully (verified)"
-        } else {
-            Write-Err "Copilot instructions deployment failed verification"
+    if ($copilotInstalled) {
+        Write-Info "GitHub Copilot CLI extension detected, deploying configuration..."
+        $CopilotHome = "$env:USERPROFILE\.copilot"
+        Ensure-Directory $CopilotHome
+
+        $copilotSource = "$DotfilesDir\ai\copilot"
+        if (Test-Path $copilotSource) {
+            Copy-Item "$copilotSource\*" "$CopilotHome\" -Recurse -Force -ErrorAction SilentlyContinue
+            if ((Test-Path "$CopilotHome\copilot-instructions.md") -and
+                (Select-String -Path "$CopilotHome\copilot-instructions.md" -Pattern 'First, read `AGENTS.md`' -SimpleMatch -Quiet)) {
+                Write-Success "copilot-instructions.md deployed successfully (verified pointer to AGENTS.md)"
+            } else {
+                Write-Warn "copilot-instructions.md deployment failed verification (expected pointer to AGENTS.md)"
+            }
         }
-    }
 
-    Write-Success "GitHub Copilot CLI configured (aliases ghcs/ghce in profile.ps1)"
+        Write-Success "GitHub Copilot CLI configured (aliases ghcs/ghce in profile.ps1)"
+    } else {
+        Write-Info "GitHub Copilot CLI extension not installed, skipping Copilot config (install with: gh extension install github/gh-copilot)"
+    }
 } else {
-    Write-Warn "GitHub CLI (gh) not found, skipping Copilot installation"
+    Write-Warn "GitHub CLI (gh) not found, skipping Copilot setup"
 }
 
 # Weekly vault maintenance scheduled task (Sundays 10:07 AM)
diff --git a/tests/verify-setup.bats b/tests/verify-setup.bats
index 455006a..a55e72e 100644
--- a/tests/verify-setup.bats
+++ b/tests/verify-setup.bats
@@ -285,8 +285,13 @@ setup() {
 # Section 10: Graceful skips (optional tools not present)
 # =============================================================================
 
-@test "copilot directory created (gh installed)" {
-    [ -d "$HOME/.copilot" ]
+@test "copilot config NOT deployed when gh-copilot extension is absent" {
+    # Post-BUG-001 (PR #40): setup-linux.sh uses detect-and-act. The
+    # gh-copilot extension is no longer auto-installed; ~/.copilot is created
+    # only if the extension is genuinely present. In the integration container
+    # gh is installed (as a dev tool) but gh-copilot is not, so the directory
+    # should NOT exist — confirming the skip path is silent and correct.
+    [ ! -d "$HOME/.copilot" ]
 }
 
 @test "no MCP servers registered (claude CLI absent)" {