fix(annotate): add /api/save-notes POST endpoint to annotate server

[titosemi]

Fixes #844

Problem

Saving annotations to Obsidian (Save to Obsidian button) fails silently in annotation mode. The button is visible and clickable but nothing happens because the annotate server lacks the /api/save-notes POST route.

The plan review server has this route and works correctly — only the annotate server was missing it.

Root Cause

packages/server/annotate.ts (Bun source) and apps/pi-extension/server/serverAnnotate.ts (Pi extension copy) did not implement the /api/save-notes handler. When the frontend sends the save request in annotation mode, the server falls through to the HTML catch-all route, returning the SPA HTML instead of JSON.

Changes

• packages/server/annotate.ts — Added /api/save-notes POST route handler with the same integration logic as serverPlan.ts: accepts obsidian, bear, and octarine configs, runs configured saves in parallel, returns { ok: true, results }.
• apps/pi-extension/server/serverAnnotate.ts — Same route added to the Pi extension copy of the annotate server.
• packages/server/integrations.test.ts — Added saveToObsidian unit tests (success and missing vault cases). Consolidated duplicate imports from ./integrations into a single statement.
• packages/server/annotate.test.ts — HTTP endpoint tests for the new route (success, empty integrations, integration-level error).

Testing

bun test packages/server/annotate.test.ts
  ✓ POST saves to Obsidian vault and returns success
  ✓ POST returns 200 with empty results when no integrations
  ✓ POST with missing vault returns integration error, not server error

bun test packages/server/integrations.test.ts
  ✓ saveToObsidian writes plan file to temp vault
  ✓ saveToObsidian fails when vault path does not exist

bun test tests/parity/route-parity.test.ts
  ✓ All Bun ↔ Pi routes match (including save-notes)

All 205 server tests pass, 0 fail.

[backnotprop]

thank you!