feat(checkpoint): add canonical restore manifest guard

[luoye520ww]

Summary

Partial implementation for the rejected KunAgent#696 checkpoint restore-manifest/canonical-path replacement.

Problem

The previous rejected work failed around macOS canonical paths, especially raw comparisons of /var/folders/... and /private/var/folders/....

Changes implemented

• Added src/main/services/git-checkpoint-manifest.ts with:
  • GitCheckpointManifestV1
  • canonical path helper using realpath()
  • manifest construction from metadata-like input
  • expected restore context validation for thread/workspace
• Added focused tests for canonical manifest helper behavior.
• Removed the non-code .kun/review-plans file from this PR scope.

Still required before this PR is reviewable

• Integrate manifest.json write/read into src/main/services/git-checkpoint-service.ts.
• Keep metadata.json compatibility for legacy checkpoints.
• Extend restore params/IPC schema with expected context if upstream wants that guard exposed.
• Add destructive-restore tests that prove wrong thread/workspace is rejected before git reset/clean.
• Add explicit macOS /var vs /private/var fixture or mock-realpath regression.

Validation to run locally

npm run typecheck
npm test

I could not run local commands from this environment, so this remains draft.