Audio: capture system audio and mic on macOS 13+ (Screen Recording permission) #15

Part of #13

Requirements: FR-07
Spec: copy specs/templates/user-story.md into specs/stories/ before starting.
ATLAS checklist: atlas/desktop.md

Acceptance criteria

Given macOS without the permission, When I start a session, Then I'm told exactly how to grant it and mic-only still works

Flow

• Specify: user story + acceptance criteria in specs/stories/
• ATLAS: review/extend atlas/desktop.md (ask the owner about any new decision)
• GOTCHA prompt written (references GOTCHA-PROJECT.md)
• Tests first, then code; coverage ≥ 75%