Remove semantic search + embeddings

[inFocus7]

note: original pr auto-closed since the underlying base pr merged

Description

Removes the unwired semantic-search / embeddings feature. The HTTP handlers, the indexer, and the public Store surface are deleted; the semantic_embedding* columns and HNSW indexes are dropped by a new v1alpha1 migration on upgrade.

What's removed:

• internal/registry/api/handlers/v0/embeddings/: the indexer kickoff and job-status endpoints. Never wired into the running router.
• internal/registry/embeddings/: the indexer, helpers, and Provider interface. Never constructed at boot.
• ⚠️ pkg/semantic/ and the v1alpha1store.Store semantic methods (SetEmbedding, GetEmbeddingMetadata, SemanticList, VectorLiteral). Public-API breaking: downstream consumers importing these symbols will get compile errors.
• EmbeddingsConfig and the AGENT_REGISTRY_EMBEDDINGS_ENABLED env var.
• The migrator's generic MigratorConfig.Skip predicate, which only ever gated 003.

Migrations:

• 003_embeddings.sql is retained as a no-op (comment-only) for historical consistency with installs that have version 203 recorded in schema_migrations. Plain postgres images can apply it.
• 008_drop_semantic_embeddings.sql drops the columns and HNSW indexes with IF EXISTS, safe on installs that never ran 003 originally.

Change Type

/kind breaking_change
/kind cleanup

Changelog

Remove embeddings and semantic search.
⚠️ If you had previously enabled embeddings and use the pgvector postgres image, keep that image in place during the upgrade migration. Once the upgrade completes, you can `DROP EXTENSION vector;` and switch to a standard Postgres image post-release.

Additional Notes

Upgrade notes

Operators currently running with AGENT_REGISTRY_EMBEDDINGS_ENABLED=true on a postgres image that includes pgvector (e.g. pgvector/pgvector:pg16) must keep the pgvector image during this upgrade.

Migration 208 drops the semantic_embedding* columns and HNSW indexes, which are typed against pgvector. PostgreSQL requires the extension's shared library to be loadable on disk to execute the drops. Attempting to upgrade the application and switch to a plain postgres image in the same Helm release fails with:

ERROR: could not access file "$libdir/vector": No such file or directory

To remove the pgvector dependency:

1. Upgrade the application first, keeping the pgvector image.
2. Verify migration 208 applied: SELECT version FROM schema_migrations WHERE version=208;
3. In a follow-up Helm release, swap the bundled postgres image to plain postgres of the same major version.

Operators who ran with EMBEDDINGS_ENABLED=false (the default) have nothing to drop and can upgrade freely on plain postgres.

[nikolasmatt]

Clean removal — no stale references in Go, charts, or docs; handler was never wired, confirmed. One important on 008, two nits.

Also: PR body's breaking-change list omits the MigratorConfig signature change (MigratorConfig(bool) → MigratorConfig()) and the removal of the exported MigratorConfig.Skip field. Downstream consumers will hit compile errors — worth listing alongside pkg/semantic.