perf: shrink the lossless search index

[davidrobertson]

Summary

• convert event, session-summary, and summary-node FTS tables to contentless-delete indexes keyed by canonical rowid
• migrate existing search indexes in one transaction with rollback validation
• defer one-time space reclamation to detached maintenance while keeping raw JSONL authoritative

Validation

• npm run typecheck
• npm test — 240 passed
• npm run smoke
• npm pack --dry-run
• real Codex CLI session loaded this plugin, wrote lifecycle events to a disposable store, and recalled four prior sessions through lcm_grep
• code review, manual QA, and final gate review passed at 2c851b197018cc9719b97c452a6ddbe5a135d5ea

Size check

A disposable reconstruction of the current local store reduced the three FTS structures from 284.9 MB to 14.1 MB. Twelve sampled top-20 ranking hashes matched. This projects about 270.8 MB less FTS storage, roughly 52% of the current 521.1 MB index.

Rollout note

Restart or refresh the installed MCP process after updating so it uses the new rowid joins.