feat: add bounded relevance-ranked search

[jmgilman]

Summary

• replace literal substring discovery with deterministic token-ranked catalog search
• add host-provided SearchTerms, bounded SearchResponse results, and truncation signaling
• cut the MCP search_api contract to a non-null object response and document search boundaries

Validation

• mise exec -- go test ./internal/catalog ./mcpserver . -count=1
• mise exec -- moon run root:check
• mise exec -- go test ./mcpserver -run ^TestActualMCPSecureLoop$ -count=1 -v

Review notes

• clean cutover; no compatibility array or search_v2 path
• no fuzzy matching, external search service, or mutable index