fix(vec): implement missing sample_db_embedding_widths (#160)

[J0nd1sk]

Closes #160.

Problem

cmd_vec_reindex (_impl.py:7855) imports sample_db_embedding_widths from agentmemory.embeddings and calls it at _impl.py:7873, but the function was never defined. brainctl vec reindex therefore crashed with:

{"error": "ImportError: cannot import name 'sample_db_embedding_widths' from 'agentmemory.embeddings' ..."}

for everyone. pyproject.toml documents vec reindex as the way to migrate the index after changing BRAINCTL_EMBED_MODEL, so that whole path was dead.

Fix

Implement sample_db_embedding_widths(conn, sample_size=8) next to get_db_embedding_dim, matching the contract the existing callsite already expects (ok, sample_count, consistent, declared_dim, observed_dims, message):

• samples up to sample_size rows from vec_memories, width = len(blob) // 4 (float32);
• cross-checks observed width(s) against the DDL-declared dim (audit H4);
• returns ok=False, sample_count=0 when the table is absent/unreadable, so the guard skips and a fresh index is built;
• flags consistent=False only on genuine disagreement (mixed widths, or uniform width ≠ declared dim).

Tests

tests/test_embeddings_width_probe.py — 9 tests: import regression, missing/empty table, uniform-matches-declared, uniform-mismatch, mixed-width corruption, unknown declared dim, sample-size cap, and a real sqlite-vec vec0 round-trip.

Verification

$ pytest tests/test_vec.py tests/test_embeddings_width_probe.py -q
30 passed

# end-to-end, against a fresh brain.db with Ollama running:
$ brainctl vec reindex
{"ok": true, "model": "nomic-embed-text", "dim": 768, "embedded": 1, ...}