docs(api-reference): add Dream (memory synthesis) endpoints

Dream shipped to prod but its API endpoints weren't in the API reference. Add
the org/project-scoped Dream endpoints (OpenAPI spec + per-endpoint pages + nav):

- GET/PATCH dream/config/  — get / update Synthesis config + plan entitlements
- GET  dream/stats/        — lifecycle + synthesis counts
- GET  dream/activity/     — supersede/merge feed (keyset-paginated)
- GET  dream/runs/         — synthesis runs feed
- GET  dream/runs/{run_id}/memories/       — memories within a run
- GET  dream/memory/{memory_id}/sources/   — a synthesized memory's sources
- POST dream/preview/      — no-write scope preview

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/api-reference/dream/dream-preview.mdx

@@ -0,0 +1,5 @@
+---
+title: "Preview Dream Scope"
+description: "A no-write preview of the scope Dream synthesis would analyze for a project."
+openapi: "post /api/v1/orgs/organizations/{org_id}/projects/{project_id}/dream/preview/"
+---