macOS Settings window: shared preferences UI and Cmd+, shortcut

[SarthakWade]

Problem

Headless has no central Settings window. Users must know individual CLI commands, while future credential and authorization policies need a trusted user-only interface.

Scope

Add a native macOS Settings window opened by Cmd+,. Render settings from the typed registry rather than maintaining a second set of defaults or validators. Group preferences by General, Browser, Agent access, Privacy, and Credentials as those settings become available.

The UI must show each setting's current value, effective default, platform scope, restart requirement, and whether agents may modify it. Include reset-to-default behavior and preserve the existing startup-presentation preference.

Boundaries

Do not expose host security invariants as toggles. Password entry uses native secure fields and never appears in generic command text, accessibility snapshots intended for agents, logs, or command history. Linux continues to use the CLI and must not pretend a GUI capability exists.

Acceptance criteria

• Cmd+, opens or focuses one Settings window.
• UI and CLI read and write the same store with identical validation.
• Changes take effect according to registry metadata and persist across restart.
• VoiceOver labels, keyboard navigation, reduced motion, and narrow-window layout are verified.
• macOS menu and E2E tests cover opening, editing, resetting, and persistence.
• No duplicate settings constants or dead compatibility paths are introduced.

Blocked by #153.

[yashranaway]

Not needed for the first credential cut.

#166 is skipped, so there is no user-only "use without confirmation" toggle to hide from the agent. Interactive CLI plus a native save sheet in #157 is enough. Reopen this when we have a real user-only policy, or when Cmd+, is worth doing on its own.

[yashranaway]

Parked. Decision 24 says interactive CLI plus a native save sheet is enough for the first credential cut. Reopen when there is a real user-only policy, or when Cmd+, is worth doing on its own. #160 can still audit shortcut collisions without this window.