Structured Worker Checkpoint Handoffs

[nathanwhitbot]

Summary

• Add compact structured worker checkpoints for durable handoff state such as current hypothesis, touched subsystems, commands run, pending checks, risks, and recommended next worker prompts.
• Parse checkpoints from worker output, preserve the human summary unchanged, and record checkpoint data on worker completion plus a dedicated task artifact.
• Project checkpoint context into later worker prompts and context ledger entries with explicit compaction bounds, while ignoring missing, empty, or malformed checkpoint data.

Validation

• go test ./internal/orchestrator -run 'Test(ParseWorkerCheckpointFromJSONEnvelope|ParseWorkerCheckpointFromFencedSnakeCaseJSON|ParseWorkerCheckpointMalformedOrEmptyIsIgnored|CompactWorkerCheckpointBoundsFields|ProjectTaskContextLedgerKeepsWorkerCheckpointAndBoundsIt|CodexBrain|ServicePersistsWorkerCheckpointAndProjectsToFollowUpPrompt)' -count=1 -v
• go test ./internal/orchestrator -count=1

[nathanwhit]

Reviewed the structured worker checkpoint handoff support. Checkpoints are parsed from final worker output, compacted before storage and prompt reuse, persisted as completion payload data plus artifacts, and projected into follow-up prompts and context ledger entries with bounds. CI is green.