Add session-level metadata (slug, version) #43

Context

RFC #41 mentioned capturing session-level metadata (slug, version) for the sessions table. The RFC implementation focused on event-level tracking for agent work, so session-level metadata was deferred.

Current State

• The Session dataclass has a slug field but update_session_stats() doesn't populate it
• Event-level version tracking was added in RFC RFC: Close data coverage gaps in session analytics ingestion #41, but not session-level

Proposed Enhancement

1. Extract slug from session log entries and populate in sessions table
2. Consider adding version to sessions table (min/max version seen, or most common)

Priority

Low - the core functionality (agent tracking, token deduplication) is complete. This is incremental metadata enrichment.

Related

• RFC RFC: Close data coverage gaps in session analytics ingestion #41: Close data coverage gaps in session analytics ingestion (RFC: Close data coverage gaps in session analytics ingestion #41)
• PR feat: Add agent tracking and fix token deduplication (RFC #41) #42: Implementation of RFC RFC: Close data coverage gaps in session analytics ingestion #41