feat: select repository by SCM rootUri when generating commit messages ✨

**✨ Features**
- Use the SCM context's rootUri to choose which repository GitUtils.getStagedDiff reads so the staged diff comes from the correct workspace.
- Prefer the repository matching the SCM context when locating the SCM input box, falling back to the first repository for backward compatibility.

**🧹 Chores**
- Add GitUtils.findRepositoryByRootUri and update getStagedDiff to accept an optional rootUri, with safer internalRepo typing and fallback behavior.
- Expose rootUri on the Repository interface to enable reliable repo matching by workspace path.

**🧪 Tests**
- Add unit tests confirming getStagedDiff uses the correct repository and that generated commit messages are written to the matching repo's input box.
- Add tests ensuring getStagedDiff falls back to the first repo when no rootUri is provided and that compactDiff removes excess context lines.