docs: Phase 781 member roster mesh sync roadmap and release notes (v0.8.748)

Document guild-sync-ready propagation, UI roster refresh, and agentctl journey.

Co-authored-by: Cursor <[email protected]>

release_notes/RELEASE_v0.8.748.md

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+# Release v0.8.748 — Member roster mesh sync (Phase 781)
+
+## Summary
+
+Member list updates reliably after guild mesh sync and paged `MEMBER_PAGE` ingest, with agentctl coverage for 2-peer roster parity.
+
+## Highlights
+
+- Worker listens for `guild-sync-ready` and pushes structural state when members arrive.
+- `MEMBER_PAGE` ingest emits `guild-sync-ready` after applying pages.
+- UI `memberRosterNeedsRefresh` updates the sidebar on light state polls.
+- Agentctl matchers: `membersMin`, `meshPeersMin`, `memberRosterSyncComplete`.
+- Journey: `runGuildMeshMemberRosterJourney`.
+
+## Verification
+
+```bash
+cd apps/pearcord
+npm run test:ci-phase781
+```