fix(store): isolate checkpoint file ownership

[DeliciousBuding]

Summary

• Detach checkpoint file lists at both write ownership boundaries so callers cannot change saved pre-run evidence through an input or returned slice.
• Cover all three real store backends, File/SQLite close-reopen durability, nil/empty writer results, and the actual checkpoint-file preview handler (including checkpoint replacement).
• Keep read/snapshot behavior, metadata, file ordering, missing-run errors, persistence policy, authorization and restore behavior unchanged. Four production lines added; no new API or storage abstraction.

Closes #2341.

Verification

Confirmed red baseline on production base b091863506538543203344be8f5bc7aae41cbcc3; green/final local evidence on 21745169df7f56b14a8583877deaa1bb3f5a624c.

Passed:

• Complete store/API tests on Windows.
• Complete store/API Linux race tests.
• Existing store/API/lifecycle checkpoint tests, including event-before-start and no-workdir absence.
• vet, staticcheck, diff whitespace checks.

All seven required CI checks passed under strict branch protection. CI run 34029282680 completed successfully, including real PostgreSQL/Redis integration and fixture backend E2E. Frontend/native/real-E2E lanes were skipped by path/dispatch rules and are not claimed as tested.

Not run or claimed: live/real-model E2E, packaged Desktop, remote source sync, rebuild, restart or runtime deployment. Existing public architecture/API documentation remains accurate; no contract documentation change is required.

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on this repository. Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Team

Run ID: 200185d9-9c0b-4265-b293-85e095ab2aa8

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}