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/writer.py

@@ -0,0 +1,43 @@
+from pathlib import Path
+
+from pyra.memory.index import update_index
+from pyra.security.boundaries import assert_safe_path
+from pyra.utils.paths import pyra_home, safe_chmod
+
+_MEMORY_ROOT = pyra_home() / "memory"
+
+
+def _resolve_and_validate(name: str) -> Path:
+    if name.startswith("/") or name.startswith("~"):
+        raise ValueError(f"Memory name must be relative, got: {name!r}")
+    path = (_MEMORY_ROOT / name).resolve()
+    assert_safe_path(path)
+    try:
+        path.relative_to(_MEMORY_ROOT.resolve())
+    except ValueError:
+        raise PermissionError(f"Path escapes memory directory: {name!r}")
+    if path.suffix.lower() != ".md":
+        path = path.with_suffix(".md")
+    return path
+
+
+def write_memory(name: str, content: str) -> Path:
+    path = _resolve_and_validate(name)
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_text(content)
+    safe_chmod(path, 0o600)
+    update_index()
+    return path
+
+
+def append_memory(name: str, content: str) -> Path:
+    path = _resolve_and_validate(name)
+    if path.exists():
+        existing = path.read_text()
+        path.write_text(existing.rstrip() + "\n\n" + content)
+    else:
+        path.write_text(content)
+        path.parent.mkdir(parents=True, exist_ok=True)
+    safe_chmod(path, 0o600)
+    update_index()
+    return path