Summary

MarkdownMemoryStorage (the deermem backend's markdown storage) silently destroys an unreadable memory.json. When the file cannot be decoded (e.g. UTF-16 BOM, invalid UTF-8, permission error), _load_summary...'s except (OSError, UnicodeError) branch logs a warning and returns None, and the next commit then rebuilds a fresh revision-1 manifest over the original bytes — the user's stored memories are gone with no backup.

The module documents a quarantine contract for exactly this case (memory.json.corrupt-<timestamp>), and the sibling decode/fence path already implements it; the summary-load except branch skips it.

Repro

1. Write a memory.json that is not valid UTF-8 (e.g. encode a valid summary as UTF-16 with BOM).
2. Load the storage (MarkdownMemoryStorage(...) + a read/load call).
3. Save anything (e.g. save(create_empty_memory())).

On current main (commit 62130caa):

WARNING ... Cannot read memory summary ...: 'utf-8' codec can't decode byte 0xff in position 0: invalid start byte
# load() returns an empty document, zero quarantine files
# save() returns True; the original bytes are absent everywhere under the storage root;
# no backup; the manifest is rewritten at revision: 1

Expected: the unreadable file is moved aside as memory.json.corrupt-* (best-effort, per the module's documented contract) and the original bytes remain recoverable; a subsequent save must not destroy them.

Suggested fix

Call the existing best-effort self._quarantine_unreadable(path) in the summary-load except (OSError, UnicodeError) branch before returning None — matching the sibling decode/fence path that already quarantines. One line; verified to preserve the original bytes (a single memory.json.corrupt-* with byte-identical content survives subsequent loads and saves; repeated loads do not create duplicates because the file is moved away).

Tests

backend/tests/test_memory_storage.py::TestMarkdownMemoryStorage::test_non_utf8_summary_is_quarantined_before_rebuild (UTF-16 BOM file; asserts one quarantine file with the original bytes that survive a subsequent save).

• On base: fails — AssertionError: a non-UTF-8 summary must be quarantined, not silently rebuilt over / assert 0 == 1.
• With the fix: passes; the six neighboring memory-backend modules are 189 passed / 1 skipped (base: 188/1, delta exactly the new test). Ruff clean.