fix(portability): refuse path traversal in import_into

[egarlian]

• import_into builds the target path as store.root / entry["path"]
  and writes to it without checking that the resolved path stays under
  the store root.
• An export payload carrying ../../etc/malicious or an absolute path
  like /tmp/escape would write outside the store directory. The
  export path cannot produce such entries (relative_to constrains
  them), but import_into is a public function that any caller can
  reach with an arbitrary payload.

Fix: resolve the target path and verify it is relative to the
resolved store root before writing. Refuse with a ValueError if the
path escapes.

target = (store.root / str(entry[KEY_PATH])).resolve()
if not target.is_relative_to(store.root.resolve()):
    raise ValueError(f"path traversal refused: {entry[KEY_PATH]}")

Files changed:

• packages/core/src/agent_memory/core/portability.py — 2-line guard
  in import_into
• tests/unit/test_portability.py — 5 tests: round-trip, file export,
  ../ traversal refusal, absolute path refusal, valid relative path

$ python3 -m pytest tests/unit/test_portability.py -v
5 passed in 0.97s