refactor(#1789): ADR-1769 Phase 5 — milestoneComplete migration (#1790)

Migrate the STATE.md write path inside cmdMilestoneComplete (milestone.cts)
onto the STATE.md Transition Module substrate (ADR-1769, epic #1769).

- Add {kind: 'milestoneComplete'} to StateTransitionIntent, with
  milestoneCompleteCore in src/state-transition.cts (consulting the
  field-classification table). Owns the closure write: Status
  ('<version> milestone complete'), Last Activity, Last Activity Description,
  a Current Position reset to 'Awaiting next milestone', and an Operator Next
  Steps reset pointing at the next-milestone command.
- Collapse the inline STATE.md transform in cmdMilestoneComplete to a
  transitionCore dispatch. The adapter retains writeStateMd (lock + steady-state
  syncStateFrontmatter post-sync) and resolves the runtime-specific
  next-milestone slash command, injecting it via intent.nextMilestoneCommand.
- The two section resets carry their pre-seam allow-adhoc-markdown waivers
  (regex semantics pinned by existing tests; pending collectSection #1372).
- realClock.today() is byte-identical to milestone.cts's local today
  (both new Date().toISOString().split('T')[0]).
- Characterization tests pin field updates, both section resets (replace +
  insert paths), and frontmatter #1255 parity.

All 66 transition + milestone + state tests pass.

Closes #1789

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

@@ -212,6 +212,6 @@ alongside, leave callbacks — parallel worlds don't converge (ADR-857's failure
 | 2 | `advancePlan` | #1782 | — |
 | 3 | `completePhase` + `phase.cts:1770` | #1784 | — |
 | 4 | `plannedPhase` + `milestoneSwitch` | #1786 | — |
-| 5 | `milestoneComplete` + `milestone.cts:352` | TBD | — |
+| 5 | `milestoneComplete` + `milestone.cts:352` | #1789 | — |
 | 6 | `patch` | TBD | #1743, #1695 |
 | 7 | `sync`, `prune`, `update` | TBD | #1760, #1761 |