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/milestone.cts

@@ -138,7 +138,7 @@ function cmdMilestoneComplete(cwd: string, version: string, options: MilestoneCo
   const milestonesPath = path.join(planningBase, 'MILESTONES.md');
   const archiveDir = path.join(planningBase, 'milestones');
   const phasesDir = planningPaths(cwd).phases;
-  const today = new Date().toISOString().split('T')[0];
+  const today = realClock.localToday();
   const milestoneName = options.name || version;
 
   // Ensure archive directory exists