refactor(#1793): ADR-1769 Phase 7 — sync, prune, update migrations (#1760, #1761) (#1794)

Migrate cmdStateSync, cmdStatePrune, cmdStateUpdate (state.cts) onto the
STATE.md Transition Module substrate and close the maintenance bug pair
#1760/#1761 (ADR-1769, epic #1769). Completes the substrate: all 10
lifecycle/maintenance transitions now route through transitionCore.

- Add {kind: 'sync'|'prune'|'update'} to StateTransitionIntent with syncCore,
  pruneCore, updateCore in src/state-transition.cts.
- updateCore: body-only single-field update (strip/reassemble), mirroring the
  pre-migration cmdStateUpdate contract.
- pruneCore: pure content→content section pruning (Decisions / Recently
  Completed / resolved Blockers / Performance Metrics rows at or below cutoff),
  byte-identical tokenizeHeadings splicing. Adapter owns currentPhase, dry-run,
  and STATE-ARCHIVE.md.
- syncCore: body writes (Total Plans in Phase, Progress bar, Last Activity)
  given disk-derived numbers. Adapter owns the disk scan + roadmap scope.
- #1760: cmdStatePrune now derives currentPhase from 'Current Phase' OR 'Phase'
  (the canonical template emits 'Phase: X of Y'), so prune engages on
  template-conformant STATE.md instead of bailing 'Only 0 phases'.
- #1761: cmdStateSync skips the Progress write (percent=null) when a milestone
  version is set in frontmatter but the ROADMAP has no versioned heading for it
  (milestone cannot be bounded). Projects without a milestone version are
  unaffected. Leaves progress untouched rather than silently writing fallback-
  derived wrong values.
- Regression: bug-1760 (prune engages on template field) + bug-1761 (sync
  leaves progress untouched when unbounded). ADR-1769 marked Accepted.

All 82 transition + 515 regression tests pass.

Closes #1793

docs/adr/1769-state-md-transition-module.md

@@ -1,7 +1,7 @@
 # ADR-1769: STATE.md Transition Module — intent-based transitions over scattered RMW callbacks
 
-- **Status:** Proposed (Phase 0); **Accepted** at Phase 7 closeout
-- **Date:** 2026-06-27 (Phase 0)
+- **Status:** Accepted (Phase 7 closeout — all 10 lifecycle/maintenance transitions migrated; bug cluster #1760/#1761/#1743/#1695/#1264/#1255/#1257/#3242 retired)
+- **Date:** 2026-06-27 (Phase 0); Phase 7 closeout 2026-06-27
 - **Issue:** [#1769](https://github.com/open-gsd/gsd-core/issues/1769) — epic
 - **Supersedes:** the policy portions of `syncStateFrontmatter` (`src/state.cts:1667–1743`),
   `readModifyWriteStateMd`'s post-sync preservation block (`src/state.cts:2008–2119`), and
@@ -214,4 +214,4 @@ alongside, leave callbacks — parallel worlds don't converge (ADR-857's failure
 | 4 | `plannedPhase` + `milestoneSwitch` | #1786 | — |
 | 5 | `milestoneComplete` + `milestone.cts:352` | #1789 | — |
 | 6 | `patch` | #1791 | #1743, #1695 |
-| 7 | `sync`, `prune`, `update` | TBD | #1760, #1761 |
+| 7 | `sync`, `prune`, `update` | #1793 | #1760, #1761 |