feat: add upstream handoff intake support to csv-wave-pipeline

- Implemented functionality to receive structured context from analyze-with-file or workflow-lite-plan via handoff.json.
- Enriched requirement prompts with implementation scope, key findings, and code anchors from handoff context.
- Loaded exploration artifacts into discoveries.ndjson to optimize codebase exploration for Wave 1 agents.
- Updated usage instructions for handoff integration in CLI commands.

Author: catlog22