PR Review Agent follow-up: feedback and improvements for the alpha agent

### Feature Description: Follow up of https://github.com/eclipse-theia/theia/pull/17298 Follow-up tracking issue for the `@pr-reviewer` agent. The agent was merged as alpha so the goal here is to collect feedback from real-world usage and track improvements. Please feel free to add your feedback below as comment! Comments on the original PR: - A public helper in `@theia/scm` for "URI for ref X at path Y", to replace the bracket-string access into `URI`'s private `codeUri` field in `user-interaction-tool.ts` (same pattern also exists in `scm-decorations-service.ts`). - Remove the old question content/renderer, since the new generic `UserInteractionTool` supersedes it. - If `@pr-reviewer` stays as a built-in agent, organize it into capabilities for easier customization (mentioned in the PR description). Additional feedback: - Flow - File handling: the agent opens a large number of files at once (e.g. many diff editors). Consider opening files lazily, reusing editors, or letting the user opt in. - UI / UX - Overflow: sub-elements do not handle overflow correctly, leading to a horizontal scroll bar in the chat. - Features / Ideas - Fast track mode: a fast track (or high-level fast track) review option would be a nice addition for quick sanity reviews of smaller PRs.