refactor(#1786): ADR-1769 Phase 4 — plannedPhase + milestoneSwitch migration (#1788)

Migrate cmdStatePlannedPhase and cmdStateMilestoneSwitch (state.cts) onto the
STATE.md Transition Module substrate (ADR-1769, epic #1769).

- Add {kind: 'plannedPhase'} and {kind: 'milestoneSwitch'} to
  StateTransitionIntent, with plannedPhaseCore and milestoneSwitchCore in
  src/state-transition.cts (consulting the field-classification table).
- plannedPhaseCore owns the template-aware Status/Last Activity updates, Total
  Plans in Phase, Last Activity Description, and the Current Position section
  (via the inlined mutateCurrentPositionForAdvance twin). The adapter keeps the
  resync:false readModifyWriteStateMd wrapper (#500 RC1).
- milestoneSwitchCore owns the new-milestone reset: rebuilt frontmatter
  (milestone/name/status='planning'/zeroed progress) + Current Position body
  reset. The adapter keeps acquireStateLock + platformWriteSync (NOT
  readModifyWriteStateMd — milestoneSwitch rebuilds frontmatter directly).
- Collapse both callbacks to transitionCore dispatches. The now-dead
  updateCurrentPositionFields helper and KNOWN_STATUS_PATTERNS import are
  removed (their behavior lives in the transition module's
  mutateCurrentPositionForAdvance).
- Characterization tests pin the template-aware preserve-authored invariant,
  Total Plans, Last Activity narrative, Current Position update, and the full
  milestone reset (frontmatter + position body + gsd_state_version preserve +
  Accumulated Context preserve).

All 58 transition + 177 state + phase + bug-2630/bug-905 regression tests pass.

Closes #1786

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

@@ -211,7 +211,7 @@ alongside, leave callbacks — parallel worlds don't converge (ADR-857's failure
 | 1 | Substrate + `beginPhase` | #1771 | #1255, #1257, #3242 |
 | 2 | `advancePlan` | #1782 | — |
 | 3 | `completePhase` + `phase.cts:1770` | #1784 | — |
-| 4 | `plannedPhase` + `milestoneSwitch` | TBD | — |
+| 4 | `plannedPhase` + `milestoneSwitch` | #1786 | — |
 | 5 | `milestoneComplete` + `milestone.cts:352` | TBD | — |
 | 6 | `patch` | TBD | #1743, #1695 |
 | 7 | `sync`, `prune`, `update` | TBD | #1760, #1761 |