feat(diff): add hunk-aware diff info and hover tooltip for modified lines

- replace old diff edit/line diff helpers with a git-style patch parser
- return richer diff metadata (change type, hunk id, old lines) for UI use
- update renderer to consume DiffInfo and keep current diff state for tooltips
- add hover tooltip that shows old hunk lines above modified sections
- remove legacy CSS ::after hover and add diff tooltip styles
- drop diff tests tied to removed APIs
- pass --acp to qwen agent by default

Author: vipmax