Add optional GitHub connection for live pull-request status and links

[lastobelus]

Outcome

Users can optionally connect GitHub so Markover's document list shows authoritative pull-request status and links directly to the associated pull request.

This extends the pull-request metadata presentation planned in #97.

Acceptance criteria

• GitHub integration is explicitly opt-in and can be connected, inspected, and disconnected from Markover's integration settings.
• Without a GitHub connection, the document list and existing local metadata behavior continue to work without errors or degraded access to reviews.
• For a review associated with a GitHub pull request, the document list shows the current lifecycle status at minimum: draft, open, merged, or closed.
• The pull-request indicator links to the canonical GitHub pull-request URL.
• Status refreshes at appropriate lifecycle moments (such as launch, foregrounding, or opening the document list) without excessive API traffic.
• Missing associations, deleted pull requests, insufficient repository access, expired authorization, network failures, and rate limits degrade clearly without blocking the review workflow.
• Credentials are stored through an appropriate secure mechanism and are never persisted in review JSON, logs, or document metadata.
• Automated coverage protects association, status mapping, disconnected behavior, refresh behavior, and failure handling.

Scope boundaries

• GitHub remains optional; Markover must not require a GitHub account.
• This issue owns GitHub-backed pull-request status and navigation, not a general GitHub client or pull-request editing workflow.
• Repository, branch, and pull-request identity should coordinate with Improve inbox / review management #97 rather than introducing a competing document-list metadata model.