fix(#3374): phase.complete stops harvesting stale body stopped_at (#3491)

* fix(#3374): phase.complete stops harvesting stale body stopped_at

Variant A: cmdPhaseComplete's adapter calls syncStateFrontmatter directly
(deliberately - STATE.md commits atomically with ROADMAP/REQUIREMENTS),
which also bypassed the #948/#1230 preservation pass every RMW write gets.
A stale body 'Stopped at:' line then silently clobbered a fresher
frontmatter stopped_at on every phase completion, with warnings: [].

Three layers close it without reversing #3517's refresh expectation:
- completePhaseCore now refreshes the body continuity line it implies
  ('Phase N complete, ready to plan Phase N+1'; ADR-2207 phrasing on the
  last phase), session-scoped via the new stateReplaceFieldInSession seam
  so a decoy bold Stopped-at line in an unrelated section cannot absorb
  the refresh. Replace-only - a layout with no session line keeps its
  shape and its frontmatter value survives via the preservation delta.
- the RMW post-sync preservation chunk (snapshots + table-driven
  applyStatePreservation + #2736 re-assert, full bodyDeltas wired) is
  extracted into the shared applyPostSyncPreservation helper; the
  phase.complete adapter and writeStateMd (milestone complete / state
  sync - the gap the closed PR #3442 review flagged) now run it too.
- cmdStateRecordSession pushed 'Stopped At' onto updated[] on any label
  MATCH, including a value already on disk - reporting a write that never
  changed a byte. It now reports only on real change, and the match is
  tracked separately so an identical value does not arm the #944 DWIM
  section rewrite (which would reset an executor-authored resume file to
  None).

* docs(#3374): backfill changeset pr field to 3491

* fix(#3374): drop the writeStateMd preservation pass - state sync's #905 contract is body-wins

CI on this PR caught what the closed PR #3442 review's MAJOR remediation
option (a) would have broken: state sync's #905 contract ('body annotation
beats existing frontmatter when both are present') is the opposite by
design - sync exists to re-derive frontmatter from the body. A blanket
applyStatePreservation pass on writeStateMd re-locked stale frontmatter
(current_phase 3 over the body's 5) on every sync.

Take the review's sanctioned option (b) instead: the scope claim is
accurate (phase.complete only) and the milestone complete / state sync
exposure is tracked as follow-up issue #3492.

---------

Co-authored-by: sim <sim@local>

.changeset/plucky-wolves-leap.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 3491
+---
+phase.complete no longer rewrites STATE.md frontmatter stopped_at with a stale body 'Stopped at:' line: the completion now refreshes the session continuity line it implies ('Phase N complete, ready to plan Phase N+1') and applies the standard field-preservation policy on its atomic commit path. state record-session no longer reports 'Stopped At' as updated when the value is already current.