refactor(core): drop legacy compaction event - #33404

Summary

• remove the unpublished compaction completion payload and its legacy decoder
• use only the current checkpoint payload as session.next.compaction.ended.1
• reset experimental event, projection, input, Context Epoch, and workspace/sync state while preserving canonical V1 session, message, and part rows

Why

The old payload cannot be truthfully converted because it lacks messageID, reason, and recent. Since the complete pre-release event/workspace state is disposable, resetting it lets the supported schema present the final payload as version 1 without compatibility machinery.

Verification

• bun run test test/database-migration.test.ts test/session-projector.test.ts test/session-runner.test.ts test/event.test.ts (146 passed)
• bun run migration --check
• bun run typecheck in packages/core
• JavaScript SDK regeneration and typecheck
• pre-push workspace typecheck (23/23 tasks)
• full Core suite: 1,005 passed; one unrelated macOS watcher assertion reports .git/HEAD as add instead of change