feat(milestones): add close/update lifecycle verbs with abandoned state

[utx0]

Resolves issue milestone-status-is-a-write-once-dead-field-with-no-close-or (major): milestone status was write-once — metta milestone create set status: open and no CLI path could ever close a milestone, edit its body, or change its target, so completed milestones permanently reported stale open state.

What changed

• Store: MilestonesStore.update(slug, patch) — read → patch → full-frontmatter Zod re-validation before any I/O → write; failing patches leave the file byte-identical. MilestonePatch supports name, target, clearTarget, status, description.
• Schema: status enum extended to open | closed | abandoned (additive; existing files unaffected).
• CLI: new metta milestone close <slug> [--abandoned] (conflict pre-check, auto-commit chore: close milestone <slug>) and metta milestone update <slug> (--name/--target/--clear-target/--description/--status, reopen via --status open); typed exit-4 JSON envelopes; shared commitMilestones helper.
• Renderers/rollup: rank-comparator sort (open first, terminal group slug-ascending — behavior-identical for open/closed-only data); shared MILESTONE_MARKERS (▸/✓/✗); status/progress render abandoned in red ✗.
• Guard/mint hooks + skill: milestone close/update join the Tier-2 blocked set with scopes minted only for metta-backlog; deployed and template copies byte-identical; metta-backlog skill gained close/update dispatch branches.

Quality evidence

• 3 parallel reviews: correctness PASS, security PASS, quality PASS_WITH_WARNINGS (single test-coverage warning, fixed in 26e0703f0).
• Full suite: 135 files, 2802 passed, 0 failed; tsc --noEmit, lint, and build clean; all 22 spec scenarios across 6 requirements have cited passing tests.
• All finalize gates green; specs merged; archived as spec/archive/2026-08-26-fix-milestone-status-write-once-dead-field-no-close/.

Risks: status: abandoned files fail validation under older metta builds (accepted one-way door); update re-serializes frontmatter, normalizing hand-edited key order/comments (accepted — hand-editing is the workflow this removes).

UAT results

Result: 7 pass / 0 fail / 36 skip (of 43 steps) — PASS
Run: 2026-08-26 · record committed as docs(fix-milestone-status-write-once-dead-field-no-close): UAT run record (b989e58) · spec/archive/2026-08-26-fix-milestone-status-write-once-dead-field-no-close/UAT.md

Skipped — needs manual acceptance

Step	Reason
1.1–1.4, 2.1–2.5, 3.1–3.3, 4.1–4.3, 6.1–6.2, 6.10–6.24	require metta milestone CLI invocations forbidden to the sandboxed UAT runner (store-level equivalents of 2.2/2.4/2.5/6.16/6.18/6.19 verified in passing steps 6.7/6.8/6.9)
5.1–5.2, 6.25–6.26	require invoking guarded verbs with/without a session credential — forbidden to the runner (covered by 309 guard-hook unit tests)

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https://claude.ai/code/session_01AA8wwFpob25iYZFCBBEtKK