feat: add self-update command and bump to 2.0.1

- Add `coinhunter update` CLI command for pipx/pip upgrade
- README: document update behavior and recommend pipx install
- Dynamic version badge with cacheSeconds=60
- Version bump: 2.0.0 → 2.0.1

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/coinhunter/__init__.py

@@ -1,3 +1,8 @@
 """CoinHunter V2."""
 
-__version__ = "2.0.0"
+try:
+    from importlib.metadata import version
+
+    __version__ = version("coinhunter")
+except Exception:  # pragma: no cover
+    __version__ = "unknown"