Reject paths outside the repo root in repo drivers and AI file tools

[nishantmonu51]

The AI file tools (read_file, write_file, develop_file) and the runtime's file APIs passed caller-supplied paths straight to repo.Get/Put/Delete, where both the file and admin drivers joined them onto the repo root with no containment check. A path with .. segments (e.g. /models/../../../../etc/passwd) could therefore read, write, or delete files outside the project root.

• Adds drivers.ResolveRepoPath, which resolves a repo-relative path against the root and errors if the result escapes it. Both repo drivers now use it everywhere they previously did filepath.Join(root, path) (Get, Hash, Stat, Put, MkdirAll, Rename, Delete).
• The AI file tools additionally reject paths containing .. segments up front, so the model gets a clear error instead of a filesystem error.
• Adds unit tests for ResolveRepoPath, traversal assertions in the shared repo driver test suite, and traversal cases for the read_file and write_file tools.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[nishantmonu51]

@codex: review

[chatgpt-codex-connector]

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[nishantmonu51]

@begelundmuller @k-anshul : can one of you review this ?