fix(index): clear the legacy embeddings table on reset and deindex

[michiot05]

what changed

index_db.reset() now deletes from the legacy embeddings vector table along with every other derived table, and index_db.deindex() removes the deleted artifact's row from it — mirroring how both functions already handle embedding_index.

why

reset()'s own docstring states the contract: it "clears every derived table — FTS indexes, embedding vectors, …" because "leaving stale rows here means semantic search can return orphaned hits after a reindex." the delete list covered embedding_index but skipped the legacy embeddings table — which is the table search_embeddings actually scans at query time and _rebuild_embeddings repopulates via INSERT OR REPLACE on every vouch index.

two user-visible consequences:

• orphaned semantic hits, permanently. delete an artifact from disk, run vouch index — the "drop and rebuild… idempotent" pass — and the deleted artifact's vector survives, so semantic search keeps returning it (snippet degraded to the bare id). the same applies to a single delete: deindex()'s docstring promises the embedding row is removed "for any kind", but it only cleared embedding_index.
• an fsck warning that can never be cleared. _check_orphan_embeddings deliberately scans both tables ("either one drifting silently breaks semantic retrieval") and flags the stale row as orphan_embedding — but the natural remedy it points the operator at, reindexing, never touched the table, so the warning was permanent.

what would break for an existing .vouch/

nothing — state.db is a derived cache and both changed functions are already destructive-by-contract on derived rows only. kbs carrying stale legacy vectors get them swept away on their next vouch index or gated delete, which is the documented behavior.

test plan

three regression tests, all failing on the previous code:

• tests/test_index.py::test_reset_clears_legacy_embeddings_table — reset() leaves embeddings empty
• tests/test_index.py::test_deindex_removes_legacy_embedding_row — deindex() removes the artifact's legacy row
• tests/test_health.py::test_fsck_orphan_embedding_cleared_by_reindex — the operator-visible symptom: an orphan_embedding finding is actually cleared by health.rebuild_index, instead of surviving it forever

validation run locally:

python -m pytest tests/ -q --ignore=tests/embeddings   # all passed
python -m mypy src                                     # no issues in 102 files
python -m ruff check src tests                         # all checks passed

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📥 Commits

Reviewing files that changed from the base of the PR and between 1c9ca7c and 5b3a6d8.

📒 Files selected for processing (4)

• CHANGELOG.md
• src/vouch/index_db.py
• tests/test_health.py
• tests/test_index.py

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