fix: await notifyCollabMutation on marks mutation routes

[tmchow]

Bug

All marks mutation routes were calling notifyCollabMutation without await:

// before — floating promise
notifyCollabMutation(slug, ...)

// after
await notifyCollabMutation(slug, ...)

The edit and edit/v2 routes already awaited it correctly. The marks routes did not.

Impact

• HTTP response was sent before the Yjs/collab layer was notified — browser clients missed live mark updates
• Errors from notifyCollabMutation became unhandled promise rejections instead of propagating to the caller

Fix

Added await to all 9 unawaited callsites in server/agent-routes.ts:

Route
marks/comment
marks/suggest-replace
marks/suggest-insert
marks/suggest-delete
marks/reject
marks/reply
marks/resolve
marks/unresolve
catch-all marks handler

No logic changes — surgical await additions only.