docs: add branch cleanup audit

docs/reports/branch-cleanup-audit.md

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+# Branch Cleanup Audit
+
+Date: 2026-05-21 (UTC)
+
+## Scope and limitation
+This report is intentionally conservative. Local branch visibility in this workspace is incomplete for cleanup decisions because it may not include the full remote GitHub branch inventory.
+
+Before any delete decision, the complete remote branch inventory must be collected and reviewed (including `codex/*` and `agent/*` branches on GitHub).
+
+## Branch inventory status
+- Local snapshot observed in this workspace is not sufficient to determine stale-branch deletion safety.
+- Remote GitHub branch inventory was not collected in this report.
+
+## Classification
+
+### Likely safe-delete candidates
+Deferred pending complete remote GitHub branch inventory and review.
+
+<!-- Example commands only (commented out):
+# gh repo view --json nameWithOwner
+# gh api repos/<owner>/<repo>/branches --paginate
+# gh pr list --state all --head <branch-name>
+-->
+
+### Manual-review candidates
+- All `codex/*` branches (manual review required after remote inventory collection).
+- All `agent/*` branches (manual review required after remote inventory collection).
+
+<!-- Example review workflow (commented out):
+# git fetch --all --prune
+# git branch -r | sed 's#^  ##'
+# gh pr list --state all --limit 200
+-->
+
+### Keep
+- Current working branch in active use (keep unless explicitly reviewed in a separate decision process).
+
+## Notes
+- No branches were deleted.
+- No PRs were closed or merged.