fix: use importlib.metadata for version to prevent mismatch with pyproject.toml

Fixes #2856

Previously __version__ was hardcoded as '0.4.1' in __init__.py while
pyproject.toml declared version '0.1.5'. This caused nanobot gateway to
report version 0.4.1 on startup while pip showed 0.1.5.

Now __version__ reads from importlib.metadata.version('nanobot-ai'),
keeping pyproject.toml as the single source of truth.

nanobot/__init__.py

@@ -2,7 +2,9 @@
 nanobot - A lightweight AI agent framework
 """
 
-__version__ = "0.1.5"
+from importlib.metadata import version as _pkg_version
+
+__version__ = _pkg_version("nanobot-ai")
 __logo__ = "🐈"
 
 from nanobot.nanobot import Nanobot, RunResult