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/index.py

@@ -1,9 +1,9 @@
 import datetime
 from pathlib import Path
 
-from pyra.utils.paths import pyra_home, safe_chmod
+from pyra.memory import _MEMORY_ROOT
+from pyra.utils.paths import safe_chmod
 
-_MEMORY_ROOT = pyra_home() / "memory"
 _INDEX_FILE = _MEMORY_ROOT / "MEMORY_INDEX.md"