Add GitHub pull request integration to the mac app

[khoi]

Summary

• Problem: Supaterm had no native pull request awareness in the macOS terminal UI, and branch-to-PR resolution could be inefficient or wrong for multi-workspace repos and fork workflows.
• Why it matters: users could not see the active PR and checks state in the sidebar, and forked repos could resolve the fork PR instead of the upstream PR.
• What changed: added a gh-backed GitHub integration with explicit availability and auth states, repo-level batched PR refresh, upstream-first multi-remote resolution, richer PR and check models, and sidebar presentation plus open-PR action.
• What did NOT change (scope boundary): no non-GitHub provider support, no migration layer, and no expansion beyond the current macOS app sidebar and settings flow.

Rationale

The implementation keeps branch watching close to each workspace but moves PR refresh to the repository level so multiple workspaces in the same repo share one GitHub lookup path. It also preserves fork correctness by preferring upstream, then origin, then other GitHub remotes, and carrying host, owner, and repo data through the lookup path so GitHub Enterprise hosts work the same way.

User-visible / Behavior Changes

• Terminal tabs can show the current branch pull request and a compact checks summary in the sidebar.
• Clicking the PR opens it.
• Settings now expose explicit GitHub integration states: disabled, checking, unavailable, unauthenticated, authenticated, and failure.
• Repositories with multiple workspaces refresh PR state in batches instead of one lookup per workspace.

Diagram (if applicable)

Before:
[branch/worktree change] -> [per-workspace probing] -> [stale or fork-mismatched PR state]

After:
[branch/worktree change] -> [workspace HEAD watcher] -> [repo-level refresh queue]
-> [upstream-first remote resolution] -> [batched gh GraphQL lookup]
-> [sidebar PR + checks state]

Human Verification

• Verified scenarios: focused SettingsFeatureTests and GithubPullRequestParsingTests, make mac-lint, and the full pre-push path: make web-check, make web-test, and make mac-test.
• Edge cases checked: unauthenticated or missing gh, GitHub CLI command failure handling, upstream-vs-fork PR selection, and settings reducer task ordering.
• What you did not verify: manual end-to-end UI interaction in the macOS app against a live GitHub repo after the final rebase.
• How can a human manually verify this: enable GitHub integration in Settings, open a repo with an upstream PR, switch branches and workspaces, confirm the sidebar updates and clears correctly, and click the PR row to open it.

[khoi]

Closing all open PRs per request.