Add hybrid repo-intelligence search lane for Prophet Understand v0

[mdheller]

Purpose

Make sherlock-search the hybrid query membrane over Prophet Understand / Repo Intelligence artifacts.

Parent epic: SocioProphet/prophet-platform#407

Required capability

Sherlock should query repo-intelligence artifacts using lexical, fuzzy, vector-ready, graph-aware, and evidence-aware ranking. The v0 lane should work with local fixtures before live estate-scale ingestion.

Initial implementation path

1. Add docs/PROPHET_UNDERSTAND_SEARCH.md.
2. Define the query contract for repo graph artifacts: nodes, edges, summaries, source anchors, provenance receipts, policy status, and validation results.
3. Add fixture queries:
   • what owns this file?
   • what depends on this contract?
   • what tests cover this node?
   • what changed in this PR impact set?
   • what policy gates touch this service?
4. Add ranking notes for lexical score, semantic/vector placeholder score, graph proximity, evidence quality, freshness, and policy risk.
5. Return answer objects with citations/source anchors, confidence, and graph path explanation.

Acceptance criteria

• Search over a fixture artifact returns explainable results with node IDs, paths, graph relationships, and provenance metadata.
• Ranking does not claim semantic certainty where no embedding/provenance exists.
• Query results preserve policy and validation status.
• The search contract can later federate across Lampstand and Sociosphere without schema rewrite.

Non-goals

• Do not reduce Sherlock to Fuse.js-only local search.
• Do not vendor Understand-Anything.
• Do not introduce opaque agent-generated answers without evidence paths.