fix(branch_sweep): protect legacy trunks by name, not by containment

The first org-wide audit found a repo carrying a `master` branch alongside
`main`. It landed in KEEP, but only because it happened to hold unique
content — the sweep protected the *default* branch and nothing else, so a
`master` fully folded into `main` after a rename is indistinguishable from
spent work and would have been deleted.

That is not an ordinary branch. Deleting it breaks every stale clone,
bookmark, CI reference and doc link still pointing at the old trunk, and
nothing about the branch's content says so — which is exactly why the
skill's own recipe has always excluded both names (`grep -vE
'^(main|master)$'`). Protecting only the default silently implemented half of
a rule that was written whole.

So `main` and `master` are now protected on NAME, whether or not either is
this repo's default, alongside whatever the default actually is.

Found by running the audit across 25 repos before deleting anything in any of
them — the value of audit-first is that it surfaces this class of thing while
it is still hypothetical. The test fails without the guard and passes with it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KwqicJpMqmcT5RVbyNwdKq

Author: claude

bin/branch_sweep.sh

@@ -19,7 +19,7 @@
 # view, so the safety gates from skills/repo_cleanup/SKILL.md are enforced here
 # rather than assumed of the caller:
 #
-#   * `main` / the default branch                       — never a candidate
+#   * `main`, `master`, and the default branch          — never a candidate
 #   * `archive/condemned/*`                             — PyAutoGut transit refs;
 #     voiding these before their sweep-after date destroys the recovery path
 #     the Gut exists to provide. The Gut voids them, not us.
@@ -156,10 +156,21 @@ default_branch=$(g symbolic-ref --quiet --short refs/remotes/origin/HEAD 2>/dev/
 default_branch="${default_branch#origin/}"
 default_branch="${default_branch:-main}"
 
+# Trunk names are protected whether or not they are THIS repo's default.
+# A repo migrated main<-master keeps the old trunk as an ordinary branch, and
+# once it is fully contained in main the sweep would happily delete it —
+# breaking every stale clone, bookmark and doc that still points at it. The
+# skill's own recipe has always excluded both names (`grep -vE
+# '^(main|master)$'`); protecting only the default silently dropped half of
+# that. Found by the first org-wide audit, where one repo's `master` survived
+# on unique content alone.
+is_trunk() { [[ "$1" == "$default_branch" || "$1" == "main" || "$1" == "master" ]]; }
+
 # --- classify ----------------------------------------------------------------
 safe=() keep=() protected=()
 while read -r b; do
-    [[ -n "$b" && "$b" != "HEAD" && "$b" != "$default_branch" ]] || continue
+    [[ -n "$b" && "$b" != "HEAD" ]] || continue
+    is_trunk "$b" && continue
     case "$b" in
         archive/condemned/*) protected+=("$b	gut-transit-ref"); continue ;;
     esac

tests/test_branch_sweep.py

@@ -163,6 +163,36 @@ def test_delete_removes_only_the_contained_branch(world):
     assert {"main", "unmerged", "open-pr-head", "archive/condemned/something"} <= remaining
 
 
+def test_a_legacy_trunk_is_protected_even_when_fully_merged(tmp_path):
+    """`master` after a main<-master migration is not an ordinary branch.
+
+    It is protected on NAME, not on containment: once folded into main it looks
+    exactly like spent work, and deleting it breaks every stale clone, bookmark
+    and doc still pointing at it. The first org-wide audit found a repo whose
+    `master` survived only because it happened to carry unique content — luck,
+    not a gate.
+    """
+    origin = tmp_path / "origin"
+    origin.mkdir()
+    subprocess.run(["git", "init", "-q", "-b", "main", str(origin)], check=True)
+    _git(origin, "config", "user.email", "t@t")
+    _git(origin, "config", "user.name", "t")
+    _commit(origin, "f", "base", "base")
+    # a legacy trunk fully contained in main — indistinguishable from spent work
+    _git(origin, "branch", "master")
+
+    clone = tmp_path / "clone"
+    subprocess.run(["git", "clone", "-q", str(origin), str(clone)], check=True)
+    bin_dir = tmp_path / "bin"
+    bin_dir.mkdir()
+    (bin_dir / "gh").write_text('#!/usr/bin/env bash\nexit 0\n')
+    (bin_dir / "gh").chmod(0o755)
+
+    assert _sweep(clone, bin_dir, mode="delete").returncode == 0
+    remaining = set(_git(origin, "for-each-ref", "--format=%(refname:short)", "refs/heads").split())
+    assert "master" in remaining, "a legacy trunk must never be swept"
+
+
 def test_refuses_to_run_without_gh(world):
     """Blind to open PRs means blind to in-flight work: refuse, do not guess."""
     clone, _, _ = world