feat: add 3-letter DOW prefix to injected timestamps

Changes [2026-01-28 20:30 EST] to [Wed 2026-01-28 20:30 EST].
Costs ~1 extra token but provides day-of-week for smaller models
that can't derive DOW from a date. Frontier models already handle
it, but this is cheap insurance for 7B-class models.

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

@@ -18,16 +18,17 @@ describe("injectTimestamp", () => {
       timezone: "America/New_York",
     });
 
-    expect(result).toMatch(/^\[2026-01-28 20:30 EST\] Is it the weekend\?$/);
+    expect(result).toMatch(/^\[Wed 2026-01-28 20:30 EST\] Is it the weekend\?$/);
   });
 
-  it("uses the same format as channel envelope timestamps", () => {
+  it("uses channel envelope format with DOW prefix", () => {
     const now = new Date();
     const expected = formatZonedTimestamp(now, "America/New_York");
 
     const result = injectTimestamp("hello", { timezone: "America/New_York" });
 
-    expect(result).toBe(`[${expected}] hello`);
+    // DOW prefix + formatZonedTimestamp format
+    expect(result).toBe(`[Wed ${expected}] hello`);
   });
 
   it("always uses 24-hour format", () => {
@@ -42,14 +43,14 @@ describe("injectTimestamp", () => {
     const result = injectTimestamp("hello", { timezone: "America/Chicago" });
 
     // 8:30 PM EST = 7:30 PM CST = 19:30
-    expect(result).toMatch(/^\[2026-01-28 19:30 CST\]/);
+    expect(result).toMatch(/^\[Wed 2026-01-28 19:30 CST\]/);
   });
 
   it("defaults to UTC when no timezone specified", () => {
     const result = injectTimestamp("hello", {});
 
     // 2026-01-29T01:30:00Z
-    expect(result).toMatch(/^\[2026-01-29 01:30/);
+    expect(result).toMatch(/^\[Thu 2026-01-29 01:30/);
   });
 
   it("returns empty/whitespace messages unchanged", () => {
@@ -65,7 +66,7 @@ describe("injectTimestamp", () => {
   });
 
   it("does NOT double-stamp messages already injected by us", () => {
-    const alreadyStamped = "[2026-01-28 20:30 EST] hello there";
+    const alreadyStamped = "[Wed 2026-01-28 20:30 EST] hello there";
     const result = injectTimestamp(alreadyStamped, { timezone: "America/New_York" });
 
     expect(result).toBe(alreadyStamped);
@@ -84,7 +85,7 @@ describe("injectTimestamp", () => {
 
     const result = injectTimestamp("hello", { timezone: "America/New_York" });
 
-    expect(result).toMatch(/^\[2026-02-01 00:00 EST\]/);
+    expect(result).toMatch(/^\[Sun 2026-02-01 00:00 EST\]/);
   });
 
   it("handles date boundaries (just before midnight)", () => {
@@ -92,19 +93,19 @@ describe("injectTimestamp", () => {
 
     const result = injectTimestamp("hello", { timezone: "America/New_York" });
 
-    expect(result).toMatch(/^\[2026-01-31 23:59 EST\]/);
+    expect(result).toMatch(/^\[Sat 2026-01-31 23:59 EST\]/);
   });
 
   it("handles DST correctly (same UTC hour, different local time)", () => {
     // EST (winter): UTC-5 → 2026-01-15T05:00Z = midnight Jan 15
     vi.setSystemTime(new Date("2026-01-15T05:00:00.000Z"));
     const winter = injectTimestamp("winter", { timezone: "America/New_York" });
-    expect(winter).toMatch(/^\[2026-01-15 00:00 EST\]/);
+    expect(winter).toMatch(/^\[Thu 2026-01-15 00:00 EST\]/);
 
     // EDT (summer): UTC-4 → 2026-07-15T04:00Z = midnight Jul 15
     vi.setSystemTime(new Date("2026-07-15T04:00:00.000Z"));
     const summer = injectTimestamp("summer", { timezone: "America/New_York" });
-    expect(summer).toMatch(/^\[2026-07-15 00:00 EDT\]/);
+    expect(summer).toMatch(/^\[Wed 2026-07-15 00:00 EDT\]/);
   });
 
   it("accepts a custom now date", () => {
@@ -115,7 +116,7 @@ describe("injectTimestamp", () => {
       now: customDate,
     });
 
-    expect(result).toMatch(/^\[2025-07-04 12:00 EDT\]/);
+    expect(result).toMatch(/^\[Fri 2025-07-04 12:00 EDT\]/);
   });
 });
 

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

@@ -53,7 +53,13 @@ export function injectTimestamp(message: string, opts?: TimestampInjectionOption
   const formatted = formatZonedTimestamp(now, timezone);
   if (!formatted) return message;
 
-  return `[${formatted}] ${message}`;
+  // Add 3-letter day-of-week for smaller models that can't derive DOW
+  // from a date. Costs ~1 token, cheap insurance.
+  const dow = new Intl.DateTimeFormat("en-US", { timeZone: timezone, weekday: "short" }).format(
+    now,
+  );
+
+  return `[${dow} ${formatted}] ${message}`;
 }
 
 /**

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("[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();