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/utils/helpers.py

@@ -55,11 +55,24 @@ def timestamp() -> str:
     return datetime.now().isoformat()
 
 
-def current_time_str() -> str:
-    """Human-readable current time with weekday and timezone, e.g. '2026-03-15 22:30 (Saturday) (CST)'."""
-    now = datetime.now().strftime("%Y-%m-%d %H:%M (%A)")
-    tz = time.strftime("%Z") or "UTC"
-    return f"{now} ({tz})"
+def current_time_str(timezone: str | None = None) -> str:
+    """Human-readable current time with weekday and UTC offset.
+
+    When *timezone* is a valid IANA name (e.g. ``"Asia/Shanghai"``), the time
+    is converted to that zone.  Otherwise falls back to the host local time.
+    """
+    from zoneinfo import ZoneInfo
+
+    try:
+        tz = ZoneInfo(timezone) if timezone else None
+    except (KeyError, Exception):
+        tz = None
+
+    now = datetime.now(tz=tz) if tz else datetime.now().astimezone()
+    offset = now.strftime("%z")
+    offset_fmt = f"{offset[:3]}:{offset[3:]}" if len(offset) == 5 else offset
+    tz_name = timezone or (time.strftime("%Z") or "UTC")
+    return f"{now.strftime('%Y-%m-%d %H:%M (%A)')} ({tz_name}, UTC{offset_fmt})"
 
 
 _UNSAFE_CHARS = re.compile(r'[<>:"/\\|?*]')