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/programming-org.lisp

@@ -140,7 +140,46 @@ Returns the filtered content as a string."
       (when (listp child)
         (let ((found (org-headline-find-by-title child title)))
           (when found (return-from org-headline-find-by-title found)))))
-    nil))
+     nil))
+
+(defun org-subtree-extract (org-content heading-name)
+  "Extracts a subtree by heading name from Org text. Returns the subtree
+content as a string (headline + body + children), or nil if not found."
+  (let* ((lines (uiop:split-string org-content :separator '(#\Newline)))
+         (target-depth nil)
+         (in-target nil)
+         (result nil))
+    (loop for line in lines
+          for trimmed = (string-trim '(#\Space) line)
+          do (let ((depth (when (uiop:string-prefix-p "*" trimmed)
+                            (length (subseq trimmed 0
+                                            (position-if (lambda (c) (not (char= c #\*)))
+                                                         trimmed)))))
+                   (headline-title (when (uiop:string-prefix-p "*" trimmed)
+                                     (string-trim '(#\* #\Space) trimmed))))
+               (when depth
+                 (when (string-equal headline-title heading-name)
+                   (setf target-depth depth in-target t))
+                 (when (and in-target target-depth
+                            (<= depth target-depth)
+                            (not (string-equal headline-title heading-name)))
+                   (return-from org-subtree-extract
+                     (format nil "~{~a~^~%~}" (nreverse result)))))
+               (when in-target (push line result))))
+    (when result
+      (format nil "~{~a~^~%~}" (nreverse result)))))
+
+(defun org-heading-list (org-content)
+  "Returns a list of all top-level heading names in Org text."
+  (let* ((lines (uiop:split-string org-content :separator '(#\Newline)))
+         (headings nil))
+    (dolist (line lines)
+      (let ((trimmed (string-trim '(#\Space) line)))
+        (when (uiop:string-prefix-p "* " trimmed)
+          (let ((title (string-trim '(#\* #\Space) trimmed)))
+            (unless (find title headings :test #'string-equal)
+              (push title headings))))))
+    (nreverse headings)))
 
 (defun org-modify (filepath old-text new-text)
   "Replaces all occurrences of OLD-TEXT with NEW-TEXT in filepath.