feat(coding): add native context compaction

[ktsoator]

Summary

• add a typed v2 append-only session transcript with message and compaction entries
• add idle-only manual context compaction that preserves recent complete user turns and full raw history
• expose compaction through the coding API and Web model settings menu without depending on harness
• require v2 session files and reject legacy message-only JSONL instead of migrating it
• persist compaction usage metadata and restore the compacted projection after restart

Testing

• go test ./...
• go test -race ./coding ./coding/compaction ./coding/store ./coding/transcript
• npm run build (web)
• npm run lint (web; one pre-existing exhaustive-deps warning in App.tsx)