revert: drop "Current Date:" label, keep [Wed YYYY-MM-DD HH:MM TZ]

Small model testing showed the label did not meaningfully help:
- Sub-3B models fail regardless of format
- 8B models untested with label specifically
- Frontier models never needed it

The bracket convention [Wed 2026-01-28 22:30 EST] matches existing
channel envelope format and is widely present in training data.
Saves ~2-3 tokens per message vs the labeled version.

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

@@ -163,7 +163,7 @@ describe("gateway agent handler", () => {
     await vi.waitFor(() => expect(mocks.agentCommand).toHaveBeenCalled());
 
     const callArgs = mocks.agentCommand.mock.calls[0][0];
-    expect(callArgs.message).toBe("[Current Date: Wed 2026-01-28 20:30 EST] Is it the weekend?");
+    expect(callArgs.message).toBe("[Wed 2026-01-28 20:30 EST] Is it the weekend?");
 
     mocks.loadConfigReturn = {};
     vi.useRealTimers();