DiscussionSyncer: accepted-answer support (GraphQL query + emit marker)

Context

Part of epic #12207. The Discussion view cannot surface accepted answers because the syncer neither queries nor emits the signal — a producer-side gap.

The Fix

Add GitHub's answer signal to FETCH_DISCUSSIONS_FOR_SYNC (per-comment isAnswer recommended; the query selects none today) and emit a parseable per-comment marker (e.g. a > [!ANSWER] callout) in the syncer comment loop, so the Discussion view can badge the accepted answer and render a distinct tag:'answer' canvas node. Gate the view affordance to category==='Q&A' (only ~2 of ~128 discussions). Extend verifyDiscussionFrontmatter only if the marker is frontmatter-based.

Acceptance Criteria

• Synced Q&A discussions carry a parseable accepted-answer marker.
• No regression for non-Q&A discussions.

Related

Epic #12207. Unblocks the Discussion view's answer affordance — the base Discussion view (sub 7) ships without it.