Chore: Define evidence gates for shared-repository deployment

[marco0560]

Summary

Define measurable, reproducible go/no-go gates for #51, the shared repository index service.

Acceptance criteria

• Feat: Shared Repository Index Service #51 gate defines MCP stability, client verification, benchmark corpus, and demonstrated multi-repository adoption thresholds.
• A failed gate produces a documented deferred issue with explicit reopening criteria.

Dependencies

• Feat: Create Agent Efficiency Benchmark Suite #53

Non-goals

• Gating similarity-index implementation.
• Adding an embedding-quality benchmark dependency to this issue.