feat: asynchronous embedding gateway with provider-agnostic backend

New file: org/system-embedding-gateway.org / lisp/system-embedding-gateway.lisp.

- Pluggable backends via *embedding-backend* hook and EMBEDDING_PROVIDER env var
- :hashing (default) — FNV-1a hashing trick, zero dependencies
- :ollama — POST /api/embeddings to local Ollama (nomic-embed-text)
- *embedding-queue* tracks pending objects; embed-all-pending drains queue
  with store-wide scan as fallback
- embed-queue-object called after ingest-ast to mark objects for embedding
- Deleted old stub system-embeddings.org (hashing-only, no provider switching)
- Exported embedding symbols from defpackage

Also:
- Added (in-package :passepartout) to system-model-router.org (was missing,
  caused CL-USER::DEFSKILL error on daemon start)
- Added system-embedding-gateway to skill-loader exclusion list
- Updated ROADMAP

org/system-model-router.org

@@ -25,6 +25,12 @@ at load time. The core iterates providers, calling the selector for each one.
 
 * Implementation
 
+** Package Context
+
+#+begin_src lisp
+(in-package :passepartout)
+#+end_src
+
 ** Configuration: Per-Slot Cascades
 
 Four env-configurable cascade variables, one per slot. Each cascade is a list