docs: add phased query recall and intent accuracy roadmap

[forhappy]

Summary

• Add a comprehensive phased technical design for improving Compass query recall, intent accuracy, and ranking quality.
• Document target architecture (intent extraction -> multi-channel recall -> ranked candidates -> intent-aware execution).
• Define 8 implementation phases with ownership, required files, acceptance criteria, and validation gates.
• Add explicit safety/rollback constraints and metrics tied to existing relevance harness.

Notes

This PR is intentionally documentation-only and scoped to implementation planning for query quality improvements.