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/cli/commands.py

@@ -549,6 +549,7 @@ def gateway(
         session_manager=session_manager,
         mcp_servers=config.tools.mcp_servers,
         channels_config=config.channels,
+        timezone=config.agents.defaults.timezone,
     )
 
     # Set cron callback (needs agent)
@@ -659,6 +660,7 @@ def gateway(
         on_notify=on_heartbeat_notify,
         interval_s=hb_cfg.interval_s,
         enabled=hb_cfg.enabled,
+        timezone=config.agents.defaults.timezone,
     )
 
     if channels.enabled_channels:
@@ -752,6 +754,7 @@ def agent(
         restrict_to_workspace=config.tools.restrict_to_workspace,
         mcp_servers=config.tools.mcp_servers,
         channels_config=config.channels,
+        timezone=config.agents.defaults.timezone,
     )
 
     # Shared reference for progress callbacks