fix(#2136): route operator-facing date fields through local clock day

clock.today() derived the calendar day by slicing a UTC ISO instant
(nowIso().split('T')[0]), so in any negative-UTC-offset zone during UTC's early
hours last_activity named a day the operator had not reached yet — ahead of
last_updated's local date, written by the same call.

- Add Clock.localToday(): host-local YYYY-MM-DD via getMonth/getDate/
  getFullYear, honoring the same GSD_NOW_MS pin as today()/nowIso().
- Route operator-facing date-only fields through localToday(): last_activity
  (state-transition ×7, state.cts), roadmap 'completed <date>' (phase.cts,
  roadmap.cts), milestone completion date (milestone.cts), todo/scaffold
  completion (commands.cts — now threaded through the realClock seam).
- Leave today() (UTC) as the source for internal/cosmetic stamps.

Closes #2136

src/phase.cts

@@ -1401,7 +1401,7 @@ function cmdPhaseComplete(cwd: string, phaseNum: string, raw: boolean): void {
   const roadmapPath = path.join(planningDir(cwd), 'ROADMAP.md');
   const statePath = path.join(planningDir(cwd), 'STATE.md');
   const phasesDir = path.join(planningDir(cwd), 'phases');
-  const today = realClock.today();
+  const today = realClock.localToday();
 
   const phaseInfoRaw = findPhaseInternal(cwd, phaseNum);
   if (!phaseInfoRaw) {