feat(memory): add JSON index and runtime memory_lookup/read/write tools

Gives Pyra an active memory brain: memory_index.json tracks summary +
keywords per file (like an inode table), and three built-in tools let
the AI look up, read, and overwrite memory mid-session. write_memory
accepts summary/keywords; update_index() merges the JSON index without
losing existing metadata.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/pyra/memory/index.py

@@ -1,22 +1,51 @@
 import datetime
+import json
 from pathlib import Path
 
 from pyra.memory import _MEMORY_ROOT
 from pyra.utils.paths import safe_chmod
 
 _INDEX_FILE = _MEMORY_ROOT / "MEMORY_INDEX.md"
+_JSON_INDEX_FILE = _MEMORY_ROOT / "memory_index.json"
+
+_EXCLUDED = {"MEMORY_INDEX.md", "memory_index.json"}
+
+
+def _load_json_index() -> dict:
+    if not _JSON_INDEX_FILE.exists():
+        return {}
+    try:
+        return json.loads(_JSON_INDEX_FILE.read_text())
+    except (json.JSONDecodeError, OSError):
+        return {}
 
 
 def update_index() -> None:
-    files = sorted(_MEMORY_ROOT.rglob("*.md"))
-    files = [f for f in files if f.name != "MEMORY_INDEX.md"]
+    existing = _load_json_index()
 
+    files = sorted(_MEMORY_ROOT.rglob("*.md"))
+    files = [f for f in files if f.name not in _EXCLUDED]
+
+    new_json: dict = {}
     rows: list[str] = []
+
     for f in files:
         rel = f.relative_to(_MEMORY_ROOT)
+        rel_key = rel.as_posix()
         category = rel.parts[0] if len(rel.parts) > 1 else "root"
-        mtime = datetime.datetime.fromtimestamp(f.stat().st_mtime).strftime("%Y-%m-%d %H:%M")
-        rows.append(f"| {rel} | {category} | {mtime} |")
+        mtime = datetime.datetime.fromtimestamp(f.stat().st_mtime)
+        mtime_str = mtime.strftime("%Y-%m-%d %H:%M")
+
+        prev = existing.get(rel_key, {})
+        new_json[rel_key] = {
+            "summary": prev.get("summary", ""),
+            "keywords": prev.get("keywords", []),
+            "modified": mtime.isoformat(timespec="seconds"),
+        }
+        rows.append(f"| {rel} | {category} | {mtime_str} |")
+
+    _JSON_INDEX_FILE.write_text(json.dumps(new_json, indent=2))
+    safe_chmod(_JSON_INDEX_FILE, 0o600)
 
     table = "\n".join(rows) if rows else "| _(no memory files)_ | — | — |"
     content = (
@@ -28,3 +57,14 @@ def update_index() -> None:
     )
     _INDEX_FILE.write_text(content)
     safe_chmod(_INDEX_FILE, 0o600)
+
+
+def update_json_entry(rel_path: str, summary: str, keywords: list[str]) -> None:
+    """Update the summary and keywords for one entry in the JSON index."""
+    index = _load_json_index()
+    entry = index.get(rel_path, {})
+    entry["summary"] = summary
+    entry["keywords"] = keywords
+    index[rel_path] = entry
+    _JSON_INDEX_FILE.write_text(json.dumps(index, indent=2))
+    safe_chmod(_JSON_INDEX_FILE, 0o600)