Move gemini-review.toml to correct location

Gemini CLI looks for custom command files in .gemini/commands/
directory, not in .github/workflows/. Moved gemini-review.toml
to the correct location and updated .gitignore to include the
commands directory while excluding other .gemini/ contents.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: hakatashi

.github/workflows/gemini-review.toml .gemini/commands/gemini-review.toml.github/workflows/gemini-review.toml renamed to .gemini/commands/gemini-review.toml

@@ -74,5 +74,6 @@ tokens.sqlite3
 !.vscode/launch.json
 
 # Gemini CLI
-.gemini/
+.gemini/*
+!.gemini/commands/
 gha-creds-*.json