fix: baseline goal token usage

[danyel117]

Summary

• persist private per-source usage trackers so pre-goal session history is excluded from goal token budgets
• keep V1 exact/estimated totals and V2 step/session totals independently baselined and reconciled monotonically
• recover from compaction and process resets without decreasing, freezing, or double-counting goal usage
• initialize V2 step accounting at goal creation and preserve compatibility with goals persisted before this change
• cover migration, malformed tracker normalization, transient dips, source ordering, V1 hooks, V2 events, and budget behavior

Validation

• bun run lint
• bun run typecheck
• bun test (203 pass)
• bun run build
• bun run pack:dry-run
• four Claude Opus review rounds; final result: no merge blocker

Closes #22