daily-fix: worktree_audit gc-wedge tier (git prune + gc.log)

Overview / Motivation

Auto-filed by /daily 2026-08-01 (route 2: behavior/logic change → independent review) from consolidated problem sweep entry C20 (miner-5 P11 + miner-4 P12; sessions 87fe9339 (#1876), f41944ea (#1906)).

Goal

Add a reviewed maintenance tier to scripts/worktree_audit.py that clears the git-gc wedge on the shared repo root: run git prune (off-peak, bounded) and remove stale .git/gc.log / .git/worktrees/*/gc.log blocker files so auto-gc re-arms; the first run doubles as the one-off unwedge.

Workflow gap

• Bug observed: Every merge/commit on the shared root now warns "There are too many unreachable loose objects; run 'git prune'" + "Automatic cleanup will not be performed until the file is removed" — seen inside #1876's merge attempts and #1906's Guard-4 refusal tool_result. Repo-level gc debt is suppressing auto-gc fleet-wide; nothing in the janitor set owns it.
• Why it is a workflow gap: worktree_audit.py is the daily worktree janitor and already runs git worktree prune, but no automation clears gc.log blocker files or runs git prune, so once the wedge appears it persists indefinitely (repo-root gc.log dated Aug 1, still present at compose time) and spams every git operation.
• Confidence: high
• verified-at-filing: probed live gc state (2026-08-02 UTC): cat .git/gc.log → "warning: There are too many unreachable loose objects; run 'git prune'" (mtime Aug 1 01:03); ls .git/worktrees/*/gc.log | wc -l → 29 worktree gc.log files, sampled content identical; git count-objects -v → 9,359 loose objects (size 97,980 KB), 48 packs, size-pack ~16 GB. grep -in 'gc\|prune' scripts/worktree_audit.py → only git worktree prune (lines 1164, 1241) — no git prune, no gc.log handling; git log --oneline --since='7 days ago' -- scripts/worktree_audit.py → 0 commits (2026-08-02 UTC).

Proposed change (refine in planning)

In scripts/worktree_audit.py (the daily cron_worktree_audit.sh entrypoint), a new tier gated on wedge evidence:

+ def gc_wedge_tier(root, apply=False):
+     # Fires only when .git/gc.log (or any .git/worktrees/*/gc.log) exists.
+     # 1. git prune --expire=2.weeks.ago   (bounded expiry; never `git gc --aggressive`)
+     # 2. remove the stale gc.log blocker files (repo root + per-worktree)
+     # 3. report loose-object count before/after; escalate if still wedged

Safety constraints the planner must keep: run only in the --apply cron path (report-only otherwise); never touch live worktrees' checked-out state; git prune is safe on concurrent committers per git's own reachability semantics but planner should confirm the flock/quiet-window story (the cron already runs at a 09:47 quiet slot); unverified hypothesis — verify at plan time: removing gc.log alone re-enables auto-gc, which may then run mid-session — prefer prune-then-clear ordering (miner-suggested mechanism; git docs to confirm at plan time).

Scope / surfaces

• Primary target: scripts/worktree_audit.py (+ scripts/cron_worktree_audit.sh if a flag is threaded)

Constraints / invariants

• Workflow-surface only — never experiment code, configs/, or tasks/.
• scripts/workflow_lint.py --check-asks passes; ruff on touched files passes.
• NEVER a destructive repo-root git op (reset --hard, checkout .) — git prune + gc.log removal only; the repo-root-branch guard invariants are untouched.
• Recursion guard: this task's session carries the workflow_fix_target Provenance line and MUST NOT auto-route its own subagents' workflow-fix candidates.

Provenance

• fingerprint: 0160c8eb7a27
• workflow_fix_target: scripts/worktree_audit.py
• origin: /daily 2026-08-01 problem sweep, CONSOLIDATED.md entry C20.