Feature/vector abstraction

[phlealr]

Notes: I tried to deal with postgres the same way the store-postgres works, not sure if it was the best way
I added 2 new params in the options besides the pre existing in openSearch one; the pg one that receives credentials and the driver one so the caller can choose what driver to use.

i made some updates on yml so let me know if i shouldn't

What's in here

• Driver abstraction — a small Driver interface (connect/close/upsert/get/query/remove/removeQuery) plus a name→driver registry. The plugin
  translates Seneca entity messages into driver calls and maps results back into entity shape.
• pgvector driver (the working local backend) — KNN via cosine distance (1 - (embedding <=> $1) score in [0,1]), equality filters
  AND-combined with similarity, parameterised SQL with an identifier guard against injection. Schema is not managed by the plugin (documented
  DDL).
• opensearch driver — kept from the original template as a second driver, proving the abstraction isn't vendor-shaped.
• Per-canon vector dim declaration (canon: { 'doc/chunk': { vector: { dim } } }) with a mismatch check on save.

Connection lifecycle fix

The plugin originally opened the DB connection from a seneca-promisify prepare hook. That caused the store's close cmd to fire spuriously
during plugin init, tearing the pool back down — so every save$/load$/list$ threw connect() has not been called. The mock-driver tests
didn't catch it (mock methods don't need a live connection) and the pgvector tests bypassed Seneca.

Now the connection is opened from the canonical store init action (seneca.add({ init: store.name, tag: meta.tag }, …)), exactly like
seneca-postgres-store: Seneca runs it during ready(), before routing store messages, so the connection is live before the first op; the
store close cmd ends the pool.

Testing

Two-level suite:

• Translation layer (test/VectorStore.test.ts) — a MockDriver verifies the Seneca↔driver translation with no backend.
• Driver integration (test/driver/PgvectorDriver.test.ts) — the pgvector SQL/KNN behaviour directly.

A local pgvector is provided via Docker Compose:

docker compose up -d
export SENECA_VECTOR_PG_URL=postgres://postgres:postgres@localhost:5432/postgres
npm test

The translation tests run with no backend; the pgvector tests skip cleanly unless SENECA_VECTOR_PG_URL is set. 52 tests pass with the DB;
25 pass + the rest skip without it.