scripts: use capnp store instead of reading markdown directly

Add store_helpers.py with shared helpers that call poc-memory commands
(list-keys, render, journal-tail) instead of globbing ~/.claude/memory/*.md
and parsing section headers.

All 9 Python scripts updated: get_semantic_keys(), get_topic_file_index(),
get_recent_journal(), parse_journal_entries(), read_journal_range(),
collect_topic_stems(), and file preview rendering now go through the store.

This completes the clean switch — no script reads archived markdown files.

scripts/retroactive-digest.py

@@ -155,24 +155,9 @@ def format_conversation(messages: list[dict]) -> str:
 
 
 def get_semantic_keys() -> list[str]:
-    """Get semantic memory file keys."""
-    keys = []
-    for md in sorted(MEMORY_DIR.glob("*.md")):
-        name = md.name
-        if name in ("journal.md", "work-queue.md", "MEMORY.md"):
-            continue
-        keys.append(name)
-        try:
-            with open(md) as f:
-                for line in f:
-                    if line.startswith("## "):
-                        slug = re.sub(r'[^a-z0-9-]', '',
-                                      line[3:].strip().lower().replace(' ', '-'))
-                        if slug:
-                            keys.append(f"{name}#{slug}")
-        except Exception:
-            pass
-    return keys
+    """Get semantic memory keys from the store."""
+    from store_helpers import get_semantic_keys as _get_keys
+    return _get_keys()
 
 
 def build_prompt(target_date: str, conversation: str,