hybrid search: vector-branch misreports learnings provenance as unknown

[edheltzel]

Source: PR #58 review (specialist agents, verified in-scope gap).

The hybrid vec-branch fetches provenance for loa/decisions/messages but not learnings — a vector-only learning match misreports known provenance as unknown in results.

Fix sketch: include the learnings provenance column in the hybrid vector-branch row fetch, mirroring the other three tables; add a test pinning a vector-only learning match reporting its real provenance.