feat: P2 — provider-agnostic embeddings + subtree loading

- system-embeddings (new skill): hashing-trick embeddings for memory-object
  vectors. Works offline with no Ollama/API dependency. Falls back to
  hashing-trick when no embedding backend is registered. Extensible —
  set *embedding-backend* to use Ollama/OpenAI/any provider.
- programming-org: add org-subtree-extract and org-heading-list for
  targeting specific headlines in Org text without loading whole files.
- core-context: add context-skill-subtree thin wrapper delegating to
  org-subtree-extract. Core stays thin — parsing lives in the skill.

lisp/core-context.lisp

@@ -37,7 +37,18 @@
          (data-dir (uiop:ensure-directory-pathname (or (uiop:getenv "PASSEPARTOUT_DATA_DIR") (namestring (merge-pathnames ".local/share/passepartout/" (user-homedir-pathname))))))
          (org-dir (merge-pathnames "org/" data-dir))
          (full-path (merge-pathnames filename org-dir)))
-    (if (uiop:file-exists-p full-path) (uiop:read-file-string full-path) nil)))
+     (if (uiop:file-exists-p full-path) (uiop:read-file-string full-path) nil)))
+
+(defun context-skill-subtree (skill-name heading-name)
+  "Reads a specific headline subtree from a skill's Org source file.
+Returns the content under HEADING-NAME (including children) as a string,
+or nil if the heading is not found."
+  (let ((full-source (context-skill-source skill-name)))
+    (unless full-source (return-from context-skill-subtree nil))
+    (if (fboundp 'org-subtree-extract)
+        (org-subtree-extract full-source heading-name)
+        ;; Fallback: no org-subtree-extract available, return full source
+        full-source)))
 
 (defun context-logs (&optional limit)
   "Retrieves the most recent lines from the harness's internal log."