refactor: use compact formatZonedTimestamp for injection

Replace verbose formatUserTime (Wednesday, January 28th, 2026 — 8:30 PM)
with the same formatZonedTimestamp used by channel envelopes (2026-01-28
20:30 EST). This:

- Saves ~4 tokens per message (~7 vs ~11)
- Uses globally unambiguous YYYY-MM-DD 24h format
- Removes 12/24h config option (always 24h, agent-facing)
- Anchors envelope detection to the actual format function — if channels
  change their timestamp format, our injection + detection change too
- Adds test that compares injection output to formatZonedTimestamp directly

Exported formatZonedTimestamp from auto-reply/envelope.ts for reuse.

src/auto-reply/envelope.ts

@@ -103,7 +103,7 @@ function formatUtcTimestamp(date: Date): string {
   return `${yyyy}-${mm}-${dd}T${hh}:${min}Z`;
 }
 
-function formatZonedTimestamp(date: Date, timeZone?: string): string | undefined {
+export function formatZonedTimestamp(date: Date, timeZone?: string): string | undefined {
   const parts = new Intl.DateTimeFormat("en-US", {
     timeZone,
     year: "numeric",