test(macos): verify native authentication, signing, and desktop integration #4358

Follow-up to #3934; coordinate manual setup documentation with #26. #3734 covers WSL server/device SSH migration and is not evidence of macOS completion.

Current state

At 4b480993994d102cec9603404950201fbbdc597e, unix/setup-git.ts selects pbcopy and open on Darwin, the personal Git config enables SSH signing with a public key, and wsl2-ssh-agent is Linux-only. The macOS installer tests verify installation, defaults, links, shell completion, and mise diagnostics. They do not prove interactive GitHub authentication, key availability, signing, or desktop launch behavior.

Acceptance criteria

• Document and verify the supported macOS SSH-agent/key-provider setup for personal Git signing and applicable SSH connections; do not infer that excluding the WSL bridge configures the native agent.
• Verify a temporary commit can be signed and verified with a disposable test identity, and document how the personal signing identity becomes available after login.
• Verify the GitHub login/refresh flow opens the native browser and copies the device code, including the clipboard failure fallback.
• Verify native editor launching for a local path containing spaces, plus the intended app/URL associations. Decide which LaunchServices settings require management and which remain manual.
• Automate command selection and failure cases with stubs; document any GUI/keychain checks that require a disposable interactive Mac session.
• Record which steps are automatic versus first-run setup, without publishing credentials or changing a developer's keychain during tests.