feat: add persistent token savings analytics with SQLite tracking #56
Description
SQLite-backed tracking. Every invocation records: timestamp, original_cmd, compressed_tokens, raw_tokens, savings_pct, duration_ms, project_path.
SQLite at ~/.cache/skim/analytics.db (WAL mode). 90-day retention. Fire-and-forget writes. New dependency: rusqlite with bundled feature.
Analytics command (name TBD - should be fun/viral/gamified). Options: skim score, skim flex, skim burn, skim wins, skim pulse.
Output: summary stats, daily breakdown, per-command breakdown. --format json for programmatic access.
Size: M
Depends on: B4-2
Blocks: B5