fix(system-prompt): hint session_status for date/time instead of embedding it

The system prompt intentionally excludes the current date/time for cache
stability (see 66eec295b). This leaves agents without date awareness,
causing wrong day-of-week claims (#1897, #1928, #2108).

Instead of reverting the cache optimization, add a one-line hint directing
agents to use session_status when they need the current date/time. This
keeps the prompt stable while teaching frontier models where to look.

Also adds a negative test ensuring the date/time is NOT re-added to the
system prompt, with comments explaining why and pointing to #3658 for the
complementary gateway-level timestamp injection approach.

Refs: #1897, #1928, #3658

src/agents/system-prompt.ts

@@ -51,7 +51,12 @@ function buildUserIdentitySection(ownerLine: string | undefined, isMinimal: bool
 
 function buildTimeSection(params: { userTimezone?: string }) {
   if (!params.userTimezone) return [];
-  return ["## Current Date & Time", `Time zone: ${params.userTimezone}`, ""];
+  return [
+    "## Current Date & Time",
+    `Time zone: ${params.userTimezone}`,
+    "If you need the current date, time, or day of week, use the session_status tool.",
+    "",
+  ];
 }
 
 function buildReplyTagsSection(isMinimal: boolean) {