docs: branch audit + cleanup plan (linear vs squash, dead-branch scan)

[drunkcoding]

Description

Adds docs/branch-maintenance.md — a point-in-time audit of every branch (local + remote) classified by how its work relates to main, with a staged deletion plan and a dead/stale scan.

Status update: the delete plan has since been executed (owner-run CLI). The doc now includes an Execution Log recording exactly what was removed. The audit-time snapshot table is retained as the record.

Motivation

The branch count had grown large and it was unclear which branches were safe to delete. PR_MERGE_POLICY.md §4 already requires "delete the PR branch once merged," but a backlog of merged-but-undeleted branches accumulated. This doc operationalizes §4 and distinguishes linear from squash merges (the repo's de-facto strategy) so cleanup proceeds with zero risk to unmerged work.

Type of Change

• docs — documentation only

File changes

File	Δ	Note
docs/branch-maintenance.md	add	Branch audit (linear/squash/unmerged) + dead-branch scan + staged delete plan + execution log

What was executed (see the doc's Execution Log)

• Deleted: 116 branches (104 remote + 12 local) — 7 linear, 51 remote + 12 local single-commit squash, 44 multi-commit squash, 2 trivial dead. Each re-verified fresh against origin/main before deletion; recovery SHAs captured.
• Held: 2 protected release/* branches (merged) and 1 local branch checked out in another worktree.
• State now: 100 remote branches remain — 2 protected-merged + 98 unmerged (24 with open PRs; 10 dead, 25 stale, 39 active). No unmerged/dead/stale branch was deleted.

Method (measured against origin/main, not local main)

• Linear-merged — ancestor of origin/main (git merge-base --is-ancestor); -d safe.
• Squash-merged — net diff already in main via synthetic full-tree commit + git cherry patch-id (≡ git-delete-merged-branches --effort=3); -D required.
• Unmerged — unique work not in main; kept.
• Dead/stale — age axis on unmerged branches (>180d dead, 90–180d stale).

Checklist

• I have read the CONTRIBUTING guide and the PR Merge Policy Contract.
• I have updated the tests (if applicable). — n/a (docs only)
• I have updated the documentation (this PR is the documentation).

PR Merge Policy Contract — pre-merge checklist

• git diff --stat origin/main reviewed; only docs/branch-maintenance.md — no unrelated files.
• Exactly one Type of Change (docs) ticked; the single file is within the docs allowlist.
• File changes table matches the diff.
• No debug_*/check_*/scratch_*/tmp_* scripts added.
• No test_*.py added inside the runtime package.
• No new BATCHGEN_* env-var debug guard.
• No leftover print() or commented-out code.
• No logs, traces, CSVs, checkpoints, wheels, or compiled artifacts staged; nothing added with -f.
• No public API change → no MODULE.md update needed.
• One concern only; diff is surgical (single doc, 2 commits: plan + execution log).
• Commits are clean: Angular format, body present, no Co-Authored-By.
• CI is green (format, lint, hygiene, tests). — pending CI