feat(settings): add git checkpoint cleanup interval

[musnows]

Summary

• add a General settings card for Git checkpoint management with cleanup interval options of 1, 2, 3, 5, and 10 days
• persist and normalize the new checkpointCleanup.intervalDays setting across main, renderer, and IPC schema validation
• periodically remove unused Git checkpoint directories from Kun's data directory when no thread data references their workspaceCheckpointId

Changes

• Cleanup is constrained to <kun dataDir>/git-checkpoints/<checkpointId> directories and skips checkpoint IDs still referenced by thread JSON/JSONL data.
• Cleanup cadence is tracked in <kun dataDir>/git-checkpoints/.cleanup.json so it does not run on every app launch.
• settings:set now accepts the new checkpointCleanup payload and rejects unsupported intervals.

Tests

• npm run test -- src/main/services/git-checkpoint-service.test.ts src/main/settings-store.test.ts
• npm run test -- src/main/ipc/register-app-ipc-handlers.test.ts src/main/settings-store.test.ts
• npm run typecheck
• git diff --check
• npm run dist:mac:arm64
• installed /Applications/Kun.app from the arm64 zip and verified http://127.0.0.1:18899/health