[Phase 11] Add GitHub integration and pull request creation

[cosentinode]

Goal

Integrate GitHub workflows through the gh CLI for the MVP.

Scope

• Detect whether gh is installed.
• Detect GitHub authentication status.
• Open a pull request for the current branch.
• Let the user choose or confirm base branch.
• Surface PR URL after creation.
• Handle missing remote, missing upstream, auth failure, and existing PR states.

Non-negotiables

• Code simplicity is a must: use gh CLI first instead of implementing OAuth/API clients.
• Security is a must: never store GitHub tokens in bitbygit; rely on gh auth and avoid logging secrets.
• Scalability is a must: wrap GitHub behavior behind an interface so direct API or other providers can be added later.

Acceptance Criteria

• Users can create a PR from the TUI after pushing a branch.
• Missing gh or auth produces clear setup guidance.
• PR creation can be part of a multi-step prompt plan.

[cosentinode]

Phase 11 has been split into focused child issues for safer PRs and review loops:

• [Phase 11] Add gh CLI boundary crate #45 Add gh CLI boundary crate
• [Phase 11] Add single-step open pr planning and execution #46 Add single-step open pr planning and execution
• [Phase 11] Add multi-step open pr prompt integration #47 Add multi-step open pr prompt integration

Planned order: #45, #46, #47. Issue #13 remains the umbrella tracker.