feat(toolbar): real-time SSE sync for created & deleted annotations

[raimannma]

What

Extends the toolbar's existing EventSource listener (which only handled annotation.updated) to also handle annotation.created and annotation.deleted, giving the toolbar live two-way sync with the server.

• created: adds the remote annotation to local state, deduped by id so the server's SSE echo of our own optimistic create isn't double-added. Placement/rearrange kinds are skipped (they have their own local state); resolved/dismissed are skipped via isRenderableAnnotation.
• deleted: removes the annotation from local state. The removal logic (feedback exit animation + placement/rearrange reverse-lookups) is factored into a shared removeAnnotationLocally helper reused by the updated handler, so annotation.updated behavior is unchanged.
• race handling: the optimistic-create id swap in addAnnotation is now dedup-aware — if the SSE created echo lands before the POST response resolves, the optimistic copy is dropped instead of renamed (which would otherwise duplicate the marker).

Server

No server changes needed: mcp/src/server/store.ts already emits annotation.created/annotation.updated/annotation.deleted, and http.ts (/sessions/:id/events) already broadcasts every event type to browser SSE clients. Only the client handler was missing.

Reimplemented, not cherry-picked

Concept from upstream benjitaylor#116 by @remorses. That PR is stale and bundles already-merged draw-mode code, and its rewritten handler dropped this fork's placement/rearrange special-casing. Reimplemented the ~50-line SSE concept cleanly against current code, keeping the fork's behavior intact.

Behavior changes to flag for maintainer

• The dedup-aware id swap is a small change to the existing create flow — worth a glance.
• SSE handlers intentionally do not fire the onAnnotationAdd/onAnnotationDelete callbacks for remote events (those callbacks fire only for local user actions, as before), avoiding a stale-closure annotationsRef. Flag if you'd prefer remote events to invoke callbacks too.

Build / test

pnpm build passes; pnpm --filter agentation test passes (103 tests, incl. 3 new for the dedup-on-created helper).

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