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.

nanobot/agent/loop.py

@@ -65,6 +65,7 @@ class AgentLoop:
         session_manager: SessionManager | None = None,
         mcp_servers: dict | None = None,
         channels_config: ChannelsConfig | None = None,
+        timezone: str | None = None,
     ):
         from nanobot.config.schema import ExecToolConfig, WebSearchConfig
 
@@ -83,7 +84,7 @@ class AgentLoop:
         self._start_time = time.time()
         self._last_usage: dict[str, int] = {}
 
-        self.context = ContextBuilder(workspace)
+        self.context = ContextBuilder(workspace, timezone=timezone)
         self.sessions = session_manager or SessionManager(workspace)
         self.tools = ToolRegistry()
         self.subagents = SubagentManager(