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/core-defpackage.org

@@ -106,9 +106,16 @@ The package definition. All public symbols are exported here.
     #:topological-sort-skills
     #:validate-lisp-syntax
     #:defskill
-    #:*skill-registry*
-    #:*scope-resolver*
-   #:skill
+       #:*skill-registry*
+       #:*scope-resolver*
+       #:*embedding-backend*
+       #:*embedding-queue*
+       #:*embedding-provider*
+       #:embed-queue-object
+       #:embed-object
+       #:embed-all-pending
+       #:embeddings-compute
+     #:skill
    #:skill-name
    #:skill-priority
    #:skill-dependencies