[codex] Add Vision output overwrite safety

[mimeding]

Summary

Adds output overwrite controls to Vision image-writing tools and tests the manifest plus runtime refusal to overwrite existing files by default.

Business Rationale

Vision workflows often produce derived artifacts from user images. Refusing implicit overwrites protects source and output files while still allowing intentional replacement when requested.

Coding Rationale

Overwrite behavior is enforced in the plugin helper that writes image outputs, keeping the safety rule close to the file mutation point. Manifest tests ensure the host and model can see the same output contract.

Expected LLM Performance Improvement

Models should choose explicit output paths, avoid destructive retries, and recover from existing-file conflicts with clear tool feedback instead of silently replacing artifacts.

Changes

• Behavior change
• UI change
• Refactor / chore
• Tests
• Docs

Test Plan

• swift test
• swift build -c release
• git diff --check