feat(config): add configurable timezone for runtime context

Add agent-level timezone configuration with a UTC default, propagate it into runtime context and heartbeat prompts, and document valid IANA timezone usage in the README.

README.md

@@ -1345,6 +1345,28 @@ MCP tools are automatically discovered and registered on startup. The LLM can us
 | `channels.*.allowFrom` | `[]` (deny all) | Whitelist of user IDs. Empty denies all; use `["*"]` to allow everyone. |
 
 
+### Timezone
+
+Time is context. Context should be precise.
+
+By default, nanobot uses `UTC` for runtime time context. If you want the agent to think in your local time, set `agents.defaults.timezone` to a valid [IANA timezone name](https://en.wikipedia.org/wiki/List_of_tz_database_time_zones):
+
+```json
+{
+  "agents": {
+    "defaults": {
+      "timezone": "Asia/Shanghai"
+    }
+  }
+}
+```
+
+This currently affects runtime time strings shown to the model, such as runtime context and heartbeat prompts.
+
+Common examples: `UTC`, `America/New_York`, `America/Los_Angeles`, `Europe/London`, `Europe/Berlin`, `Asia/Tokyo`, `Asia/Shanghai`, `Asia/Singapore`, `Australia/Sydney`.
+
+> Need another timezone? Browse the full [IANA Time Zone Database](https://en.wikipedia.org/wiki/List_of_tz_database_time_zones).
+
 ## 🧩 Multiple Instances
 
 Run multiple nanobot instances simultaneously with separate configs and runtime data. Use `--config` as the main entrypoint. Optionally pass `--workspace` during `onboard` when you want to initialize or update the saved workspace for a specific instance.