- Introduce baseCommitSha on sessions for task-scoped file diffs independent of shared worktrees
- Add /api/sessions/:id/files and /api/sessions/:id/file-diffs routes using baseCommitSha
- Update TerminalModal to consume task-scoped file list and diff endpoints
- Add regression tests for shared worktree scenario (multiple tasks in same worktree)
- Expand route tests for file diffs and session files with edge cases
- Clean up stale TerminalModal test helpers and simplify diff display styles
- Update README and route documentation for task-scoped diffing
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