- Remove all file revert/delete logic from check_plan_approval_gate:
no more git checkout or fs::remove_file calls. The gate only warns.
- Remove reverted_files field from ApprovalGateResult::Blocked.
- Add get_dirty_files() helper to snapshot dirty files as a HashSet.
- Capture baseline dirty files when plan mode starts (set_plan_mode).
Pre-existing dirty files are excluded from gate checks so they
never trigger blocking.
- Add 5 new unit tests covering non-destructive behavior, baseline
exclusion, and mixed baseline/new file scenarios.
- Update integration test to match new non-destructive semantics.
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