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/gateway/server-methods/agent.test.ts

@@ -125,7 +125,6 @@ describe("gateway agent handler", () => {
       agents: {
         defaults: {
           userTimezone: "America/New_York",
-          timeFormat: "12",
         },
       },
     };
@@ -164,9 +163,7 @@ describe("gateway agent handler", () => {
     await vi.waitFor(() => expect(mocks.agentCommand).toHaveBeenCalled());
 
     const callArgs = mocks.agentCommand.mock.calls[0][0];
-    expect(callArgs.message).toMatch(
-      /^\[.*Wednesday.*January 28.*2026.*8:30 PM.*\] Is it the weekend\?$/,
-    );
+    expect(callArgs.message).toBe("[2026-01-28 20:30 EST] Is it the weekend?");
 
     mocks.loadConfigReturn = {};
     vi.useRealTimers();