feat(date-time): standardize time context and tool timestamps

src/config/types.agent-defaults.ts

@@ -103,6 +103,8 @@ export type AgentDefaultsConfig = {
   bootstrapMaxChars?: number;
   /** Optional IANA timezone for the user (used in system prompt; defaults to host timezone). */
   userTimezone?: string;
+  /** Time format in system prompt: auto (OS preference), 12-hour, or 24-hour. */
+  timeFormat?: "auto" | "12" | "24";
   /** Optional display-only context window override (used for % in status UIs). */
   contextTokens?: number;
   /** Optional CLI backends for text-only fallback (claude-cli, etc.). */