refactor(memory): centralize _MEMORY_ROOT; fix mkdir order in append_memory

_MEMORY_ROOT was defined independently in reader.py, writer.py, and
index.py. Moved to memory/__init__.py; all three import from there.

Also fixes a bug in append_memory where path.write_text() was called
before path.parent.mkdir(), which would crash when creating a file in
a new subdirectory.

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

src/pyra/memory/writer.py

@@ -1,10 +1,9 @@
 from pathlib import Path
 
+from pyra.memory import _MEMORY_ROOT
 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"
+from pyra.utils.paths import safe_chmod
 
 
 def _resolve_and_validate(name: str) -> Path:
@@ -32,12 +31,12 @@ def write_memory(name: str, content: str) -> Path:
 
 def append_memory(name: str, content: str) -> Path:
     path = _resolve_and_validate(name)
+    path.parent.mkdir(parents=True, exist_ok=True)
     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