feat(tui): AuthOverlay fragment for device-flow prompt

[cdhagmann]

Overlay fragment that fires on :auth_required messages. Walks the user through the GitHub device flow and retries the original action on success.

Scope:

• GemContribute::TUI::AuthOverlay Rooibos fragment
• Triggered by Failure(:unauthenticated) from the service layer (per ADR-0012)
• Polls device-flow endpoint via Command.http + Command.wait
• On success, closes overlay and retries the pending action
• Pure-function Update test for the auth state machine

Sub-issue of #2.

See: ADR-0001, ADR-0004, docs/design.md TUI section, ROADMAP Phase 3