fix(dev): fill sync.* config placeholders in backend bootstrap

[tyler-dane]

Summary

While setting up the development environment I found that .cursor/bootstrap-backend.sh was stale relative to the current compass.example.yaml. The example config now ships a required sync: block with REPLACE_WITH_SYNC_MONGO_PASSWORD and REPLACE_WITH_SYNC_INTERNAL_AUTH_TOKEN placeholders, but the bootstrap script only filled the older backend, supertokens, and mongo.uri values. The config validator (packages/core/src/config/compass.config.ts) rejects any remaining REPLACE_WITH_ value, so both bun run dev:web and bun run dev:backend aborted at startup with:

Compass config file /workspace/compass.yaml contains unfilled placeholder values.
Replace the following fields with real values:
  - sync.mongoUri
  - sync.internalAuthToken

This PR:

• Fills sync.mongoUri (an isolated compass_sync database on the same local single-node replica set) and sync.internalAuthToken in bootstrap-backend.sh, reading optional SYNC_MONGO_URI / SYNC_INTERNAL_AUTH_TOKEN env overrides with safe local defaults.
• Documents the new env vars, the sync.mongoUri replica-set requirement, and the expected/harmless sync-change-feed ... unavailable warnings (logged when the optional dev:sync service isn't running) under ## Cursor Cloud specific instructions in AGENTS.md.

No application code changed; compass.yaml remains gitignored.

Simplicity

Minimal: two additional sed substitutions plus one default variable, mirroring the existing pattern in the script. Documentation updates only in AGENTS.md.

Automated validation

• bun lint — passes (12 pre-existing warnings, exit 0).
• bun test:core — 549 pass.
• bun test:web — 2173 pass.
• bun test:backend — 300 pass, 1 skip (in-memory Mongo auto-started).
• bun run dev:backend — GET /api/health returns 200 {"status":"ok"} with Mongo connected.
• bun run dev:web — serves http://localhost:9080 (HTTP 200).
• Manual: created a calendar event "Hello World Compass" in the web UI (anonymous/IndexedDB mode); it persists on the grid.

Independent review

Verified fresh-path bootstrap fills all sync.* placeholders (only commented google/email/posthog placeholders remain, which the validator ignores) and that re-running the script is idempotent (leaves an existing compass.yaml untouched).

Test plan

bun install
bun lint
bun test:core
bun test:web
bun test:backend
bash .cursor/bootstrap-backend.sh
bun run dev:backend   # curl -s localhost:3000/api/health -> {"status":"ok"}
bun run dev:web       # http://localhost:9080 -> 200