feat(sessions): BM25-ranked session search with search-index feature Refs #758

[AlexMikhalev]

Summary

Replaces brute-force substring matching in session search with BM25 (Okapi) scoring for relevance-ranked results, reusing the existing BM25 infrastructure from terraphim_types::score.

Changes

Step 1: Extract score module to terraphim_types

• Moved BM25/BM25F/BM25Plus/TF-IDF/Jaccard/QueryRatio scorers from terraphim_service::score to terraphim_types::score
• All 19 score tests pass in new location

Step 2: Re-export from terraphim_service

• terraphim_service::score is now a thin re-export shim: pub use terraphim_types::score::*
• Removed ~3400 lines of duplicated code, backwards compatible

Step 3: search-index feature for terraphim_sessions

• New search-index feature flag depending on terraphim_types
• session_to_document() adapter converts Session → Document (title + body from messages + metadata)
• search_sessions() performs BM25-ranked search with relative score threshold filtering

Step 4: Replace brute-force search

• SessionService::search() now uses BM25 when search-index feature is enabled
• Falls back to substring matching when feature is disabled (feature-gated with #[cfg])

Step 5: /sessions index CLI command

• Added SessionsSubcommand::Index { verbose } with parser
• Shows session count, message count, scorer type, and optional source breakdown

Testing

• 61 tests pass with search-index feature
• 52 tests pass without (fallback path)
• All pre-commit checks pass (fmt, clippy, build, tests, UBS)

Refs #758 (Gitea)