chore(maintenance): on-demand stale-branch audit script (ADR-0921)

[lusoris]

Summary

Adds scripts/maintenance/prune-stale-branches.sh — read-only audit by
default with explicit --delete / --delete-stale opt-in flags. Classifies
every remote branch on origin (248 today) by age, PR status, and
naming convention; never touches master or release-please--branches--master*.

Today's audit findings:

Bucket	Count
Total branches	248
KEEP-tier	246
DELETE-MERGED	2
DELETE-WT-OLD	0
DELETE-STALE	0
STALE-CLOSED	0

The 2 DELETE-MERGED survivors are feat/cuda-resolution-dispatch-scaffold-20260529
(PR #91 merged 2026-05-29) and feat/vmafx-tune-go-stage3-20260528 (PR #57 merged
2026-05-28) — GitHub auto-delete-on-merge stragglers. Safe to delete via
--delete after this PR merges.

Git LFS posture confirmed correctly disabled in .gitattributes (intentional —
ONNX blobs ship inline per the 2026-05-15 rationale; see ADR-0921).

Also fixes a pre-existing ADR-index drift: ADR-0887's fragment was missing,
causing every README regen to emit a WARNING.

Type

• chore — maintenance tooling

Checklist

• Commits follow Conventional Commits.
• pre-commit run --files <staged> is green locally.
• No SIMD/GPU code path touched.
• No new .c / .cpp / .cu / .h (no copyright-header requirement).

Bug-status hygiene (ADR-0165)

• docs/state.md updated — no state delta: pure tooling addition, no bug opened/closed/ruled-out.

Netflix golden-data gate (ADR-0024)

• No assertAlmostEqual modified.

Deep-dive deliverables (ADR-0108)

• Research digest — docs/research/stale-branches-audit-2026-05-31.md.
• Decision matrix — ## Alternatives considered in ADR-0921.
• AGENTS.md invariant note — added to scripts/AGENTS.md (Governing ADRs section, plus maintenance/ row in Scope tree).
• Reproducer / smoke-test command — see "Reproducer" below.
• CHANGELOG fragment — changelog.d/changed/stale-branches-audit.md.
• Rebase note — appended to docs/rebase-notes.md under "Stale-branch audit script (2026-05-31)".

Reproducer

bash scripts/maintenance/prune-stale-branches.sh           # read-only audit
bash scripts/maintenance/prune-stale-branches.sh --json    # machine-readable
bash scripts/maintenance/prune-stale-branches.sh --delete  # delete DELETE-tier

Expected output today: 248 branches scanned, 2 DELETE-MERGED candidates,
0 strict-stale.

Touched-file cleanup (CLAUDE.md §12 r12)

Fixed in this PR: missing index fragment for ADR-0887 (pre-existing
WARNING from scripts/docs/concat-adr-index.sh).