[Usability] Add contributor guide for updating TUI insta snapshots

[github-actions]

Summary

Contributors often hit failing snapshot tests in codex-rs/tui. Add a concise guide to list, review, and accept insta updates.

Current vs Expected

• Current: No single contributor‑facing doc with cargo insta commands.
• Expected: A short section under Contributing covering: run tests, view pending, review specific *.snap.new, accept intentionally, install cargo-insta if needed.

Proposed Changes

• Add a subsection to docs/contributing.md or a new docs/testing.md covering:
  • cargo test -p codex-tui
  • cargo insta pending-snapshots -p codex-tui
  • cargo insta show -p codex-tui path/to/file.snap.new
  • cargo insta accept -p codex-tui
• Link from codex-rs/tui/README or codex-rs/README.

Acceptance Criteria

• Contributor doc exists and links are discoverable from README.
• Commands verified locally.

References

• codex-rs/tui (snapshot tests via insta)