fix(consolidation): prevent orphan observations when source deleted mid-consolidation

[nicoloboschi]

Summary

Closes a race between consolidation and memory deletion where an observation could be written referencing an already-deleted source memory.

The race: consolidation fetches source memory X, runs an LLM for several seconds, then inserts an observation with source_memory_ids = [X, ...]. If X is hard-deleted during the LLM call, the delete path's stale-observation sweep has already run and cannot see the not-yet-inserted row, leaving an orphan. source_memory_ids is a uuid[] column, so Postgres cannot cascade through it — cleanup is manual by necessity, which is why this is a latent bug in the current design.

Fix (two coordinated changes):

• consolidator.py: new _filter_live_source_memories helper runs SELECT ... FOR SHARE against memory_units inside the same transaction as the INSERT/UPDATE, dropping uuids whose rows have been deleted and blocking concurrent deletes until the write commits. _create_observation_directly and _execute_update_action call it before writing; if no live sources remain, the action is skipped ({"action": "skipped", "reason": "sources_deleted"} for creates; no-op return for updates).
• memory_engine.py: delete_memory_unit, delete_document, and delete_bank(fact_type=...) now DELETE the source rows before calling _delete_stale_observations_for_memories, so under READ COMMITTED the post-delete sweep also catches any observation a concurrent consolidation committed in the window.

Together these give us: either the consolidator sees the source gone and filters it, or the delete sees the new observation and sweeps it.

Three regression tests in test_observation_invalidation.py exercise the consolidator helpers directly:

• mixed live/dead source_memory_ids → observation created with only live sources
• all-dead source_memory_ids → create skipped, no row written
• all-dead new sources on update → update skipped, observation untouched

Test plan

• uv run pytest tests/test_observation_invalidation.py — 8/8 pass locally (3 new + 5 existing)
• ./scripts/hooks/lint.sh — all passed
• CI green