feat: external annotations API with real-time SSE

[backnotprop]

Summary

• Adds a general-purpose External Annotations API so any external program can push annotations into a live Plannotator review session via HTTP and have them appear in the browser UI in real-time via Server-Sent Events
• Shared core store logic in packages/shared/ ensures identical behavior between Bun and Pi servers — both are thin HTTP adapters over the same runtime-agnostic code
• Frontend hook uses EventSource as primary transport with automatic version-gated polling fallback for restricted environments
• Includes interactive sandbox test (tests/manual/local/test-external-annotations.sh) that demos all operations: single add, batch add, delete, clear — streaming live to the browser

API

Route	Method	Purpose
/api/external-annotations/stream	GET	SSE stream — snapshot on connect, live events
/api/external-annotations	GET	JSON snapshot with ?since=N version gating (304 when unchanged)
/api/external-annotations	POST	Add single or batch ({ annotations: [...] }). Returns { ids }
/api/external-annotations	DELETE	By ?id=, ?source=, or clear all

New files

• packages/shared/external-annotation.ts — types, store, validation, SSE helpers
• packages/server/external-annotations.ts — Bun handler
• apps/pi-extension/server/external-annotations.ts — Pi handler (node:http)
• packages/ui/hooks/useExternalAnnotations.ts — EventSource + polling fallback
• packages/ui/components/ExternalAnnotationCard.tsx — card with kind badges, source labels
• tests/manual/test-external-annotations.ts — sandbox demo script

Coexistence

Existing VS Code editor annotations (/api/editor-annotation) are untouched — both systems run side by side.

Test plan

• Run ./tests/manual/local/test-external-annotations.sh — watch annotations arrive in real-time over 6 waves
• Verify SSE reconnection: kill curl stream, reconnect, get fresh snapshot
• Verify polling fallback: disable EventSource in dev tools, annotations still appear
• Verify existing /api/editor-annotation endpoints still work
• bun run build:hook && bun run --cwd apps/review build — clean builds
• Pi build: bun run --cwd apps/pi-extension build — generated/external-annotation.ts created