feat: promote GitHub repository workspace

[777lotto]

Promotion

Promote the completed GitHub-aware repository workspace from persistent bluff into production bet.

Included

• responsive, semantically highlighted, scrollable key guide
• first-class Changes, History, Actions, Issues, and Pull Requests views
• asynchronous read-only GitHub synchronization with per-repository/view caching
• authenticated gh, hardened curl, GitHub Enterprise, and short-lived token-provider support
• provider, redaction, renderer, stale-response, and Git fixture coverage
• updated README, help, changelog, security guidance, contribution guidance, and CI

Completion

• Implements RFC discussion RFC: GitHub-aware repository tabs and a clearer key guide #6.
• Promotes the exact merge from PR feat: add GitHub repository tabs and redesign the key guide #7; that PR has no review threads or requested changes.
• Closes feat: add GitHub repository tabs and redesign the key guide #5 now that the work reaches the default branch.

Validation

• GitHub Actions passed for PR feat: add GitHub repository tabs and redesign the key guide #7 on Linux with Neovim 0.10.4, 0.11.7, and 0.12.4, macOS with Neovim 0.12.4, and the quality job.
• Local Lua compilation, focused unit tests, isolated Git fixture smoke tests, and git diff --check pass on the merged bluff tree.

Closes #5