feat(memory): sandboxed markdown memory system

- memory/index.py: auto-regenerate MEMORY_INDEX.md on every write
- memory/reader.py: list_memories(), read_memory(), load_context_for_session()
  all go through assert_safe_path() + relative_to check
- memory/writer.py: write_memory(), append_memory() — relative names only,
  no absolute paths or traversal, calls update_index() after every write

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

src/pyra/memory/index.py

@@ -0,0 +1,30 @@
+import datetime
+from pathlib import Path
+
+from pyra.utils.paths import pyra_home, safe_chmod
+
+_MEMORY_ROOT = pyra_home() / "memory"
+_INDEX_FILE = _MEMORY_ROOT / "MEMORY_INDEX.md"
+
+
+def update_index() -> None:
+    files = sorted(_MEMORY_ROOT.rglob("*.md"))
+    files = [f for f in files if f.name != "MEMORY_INDEX.md"]
+
+    rows: list[str] = []
+    for f in files:
+        rel = f.relative_to(_MEMORY_ROOT)
+        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} |")
+
+    table = "\n".join(rows) if rows else "| _(no memory files)_ | — | — |"
+    content = (
+        "# Memory Index\n\n"
+        "| File | Category | Last Modified |\n"
+        "|------|----------|---------------|\n"
+        f"{table}\n\n"
+        "_Auto-maintained by Pyra. Do not edit manually._\n"
+    )
+    _INDEX_FILE.write_text(content)
+    safe_chmod(_INDEX_FILE, 0o600)