docs: add worktree-first editing policy guidance

[shunk031]

Summary

• add a worktree-first editing policy to the Codex and exact agents instructions
• translate the Worktree Policy section in the Codex and exact agents instructions into Japanese while preserving the original meaning
• require the GitHub workflow manager to treat the default branch checkout as read-only for repo-tracked files and clarify when task-relevant files should be copied into a fresh task worktree

Testing

• not run locally (documentation-only change)
• GitHub Actions re-run after the latest push

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.74%. Comparing base (af19c7f) to head (b532e6a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #440      +/-   ##
==========================================
+ Coverage   87.93%   88.74%   +0.81%     
==========================================
  Files           9        9              
  Lines         232      231       -1     
==========================================
+ Hits          204      205       +1     
+ Misses         28       26       -2     

Flag	Coverage Δ
macos-14-client	41.66% <ø> (ø)
ubuntu-latest-client	88.74% <ø> (+0.81%)	⬆️
ubuntu-latest-server	88.31% <ø> (+0.43%)	⬆️

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