refactor: rename update command to upgrade

- Align CLI verb with pipx/pip terminology (`pipx upgrade`).
- Rename internal `self_update` to `self_upgrade` for consistency.
- Update README and tests accordingly.
- Bump version to 2.0.4.

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

README.md

@@ -91,11 +91,11 @@ coinhunter opportunity portfolio
 coinhunter opportunity scan
 coinhunter opportunity scan --symbols BTCUSDT ETHUSDT SOLUSDT
 
-# Self-update
-coinhunter update
+# Self-upgrade
+coinhunter upgrade
 ```
 
-`update` will try `pipx upgrade coinhunter` first, and fall back to `pip install --upgrade coinhunter` if pipx is not available.
+`upgrade` will try `pipx upgrade coinhunter` first, and fall back to `pip install --upgrade coinhunter` if pipx is not available.
 
 ## Architecture