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-skills.org

@@ -187,7 +187,8 @@ Both ~.org~ and ~.lisp~ files are included. For each skill, the ~.org~ file supp
                                     (string= n "core-loop-act")
                                     (string= n "core-loop")
                                     (string= n "core-manifest")
-                                    (string= n "security-dispatcher"))))
+                                    (string= n "security-dispatcher")
+                                    (string= n "system-embedding-gateway"))))
                            all-files))
         (adj (make-hash-table :test 'equal))
         (name-to-file (make-hash-table :test 'equal))