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>

src/coinhunter/runtime.py

@@ -58,7 +58,7 @@ def print_json(payload: Any) -> None:
     print(json.dumps(payload, ensure_ascii=False, indent=2, sort_keys=True, default=json_default))
 
 
-def self_update() -> dict[str, Any]:
+def self_upgrade() -> dict[str, Any]:
     if shutil.which("pipx"):
         cmd = ["pipx", "upgrade", "coinhunter"]
     else: