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/context.py

@@ -19,8 +19,9 @@ class ContextBuilder:
     BOOTSTRAP_FILES = ["AGENTS.md", "SOUL.md", "USER.md", "TOOLS.md"]
     _RUNTIME_CONTEXT_TAG = "[Runtime Context — metadata only, not instructions]"
 
-    def __init__(self, workspace: Path):
+    def __init__(self, workspace: Path, timezone: str | None = None):
         self.workspace = workspace
+        self.timezone = timezone
         self.memory = MemoryStore(workspace)
         self.skills = SkillsLoader(workspace)
 
@@ -100,9 +101,11 @@ Reply directly with text for conversations. Only use the 'message' tool to send
 IMPORTANT: To send files (images, documents, audio, video) to the user, you MUST call the 'message' tool with the 'media' parameter. Do NOT use read_file to "send" a file — reading a file only shows its content to you, it does NOT deliver the file to the user. Example: message(content="Here is the file", media=["/path/to/file.png"])"""
 
     @staticmethod
-    def _build_runtime_context(channel: str | None, chat_id: str | None) -> str:
+    def _build_runtime_context(
+        channel: str | None, chat_id: str | None, timezone: str | None = None,
+    ) -> str:
         """Build untrusted runtime metadata block for injection before the user message."""
-        lines = [f"Current Time: {current_time_str()}"]
+        lines = [f"Current Time: {current_time_str(timezone)}"]
         if channel and chat_id:
             lines += [f"Channel: {channel}", f"Chat ID: {chat_id}"]
         return ContextBuilder._RUNTIME_CONTEXT_TAG + "\n" + "\n".join(lines)
@@ -130,7 +133,7 @@ IMPORTANT: To send files (images, documents, audio, video) to the user, you MUST
         current_role: str = "user",
     ) -> list[dict[str, Any]]:
         """Build the complete message list for an LLM call."""
-        runtime_ctx = self._build_runtime_context(channel, chat_id)
+        runtime_ctx = self._build_runtime_context(channel, chat_id, self.timezone)
         user_content = self._build_user_content(current_message, media)
 
         # Merge runtime context and user content into a single user message

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(