Problem
When a user's fork diverges from upstream, the web UI gives no indication. Users may unknowingly edit stale content or miss upstream updates. ADR-0018 specifies divergence indicators but they haven't been built.
Solution
Backend
Add `get_fork_divergence(repo_name)` to `RepoService` — compares fork HEAD vs upstream HEAD, returns counts of entries ahead/behind/diverged
Add `GET /api/repos/{name}/divergence` endpoint returning `{ahead: N, behind: N, diverged_entries: [...]}`
Per-entry comparison: diff fork version vs upstream version for a given entryWeb UI Indicators
Entry list: Badge showing "N entries differ from upstream" on KB cards
Entry view: Banner when user's version differs: "Your version differs from upstream. [View upstream] [Submit PR]"
KB detail page: Sync status indicator (up-to-date, behind, ahead, diverged)
Header/sidebar: Fork sync state iconDiff View
Side-by-side comparison of user's entry vs upstream version
Accessible from the divergence banner on entry pagesPrerequisites
Per-user fork directories (or current shared fork model)
GitHub integration (completed)