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/clock.cts

@@ -20,6 +20,8 @@ export interface Clock {
   now(): number;
   nowIso(): string;
   today(): string;
+  /** Host-local calendar day as YYYY-MM-DD — for operator-facing date-only fields. */
+  localToday(): string;
   sleep(ms: number): void;
 }
 
@@ -87,6 +89,26 @@ export const realClock: Clock = {
     return this.nowIso().split('T')[0];
   },
 
+  /**
+   * Return today's date as a YYYY-MM-DD string in the HOST-LOCAL calendar day.
+   * Uses this.now() so the subprocess time-pin adapter (GSD_NOW_MS) is honoured
+   * exactly as today()/nowIso() are — deterministic when GSD_NOW_MS and TZ are
+   * both pinned (#2136).
+   *
+   * Operator-facing date-only fields (last_activity, "completed <date>", etc.)
+   * must use the local calendar day: an operator reads them as "the day I did
+   * this", and they must never name a day ahead of `last_updated`'s local date.
+   * `today()` (UTC) stays the source for internal/cosmetic stamps.
+   *
+   * @returns e.g. "2020-06-14" (local), which may differ from today() near UTC midnight
+   */
+  localToday(): string {
+    const d = new Date(this.now());
+    const mm = String(d.getMonth() + 1).padStart(2, '0');
+    const dd = String(d.getDate()).padStart(2, '0');
+    return d.getFullYear() + '-' + mm + '-' + dd;
+  },
+
   /**
    * Synchronous sleep via Atomics.wait.
    * This is the identical primitive acquireStateLock and withPlanningLock used