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>

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "coinhunter"
-version = "2.0.3"
+version = "2.0.4"
 description = "Binance-first trading CLI for balances, market data, opportunity scanning, and execution."
 readme = "README.md"
 license = {text = "MIT"}