WIP Banzai Codes: add handoff, candidate-summary and PRD actions

[madsbf]

Description

Adds a new banzai-codes/ folder with three composite actions supporting the Banzai Codes refinement loop. A work order is one GitHub issue on two Projects v2 boards (matching the banzai-codes contracts): the Product board tracks the user-facing lifecycle (Define → In Progress → Acceptance → Done) and the Development (harness) board tracks the build pipeline (Ready / Todo / In Progress / Human Review / Merging / Rework / Done). No issues are duplicated; the actions only move board memberships, statuses and comments.

• banzai-codes/handoff-work-orders (no LLM): one idempotent scan of the Product board. Every open work order in In Progress that isn't on the harness board yet is added there at Todo, where banzai-codes-worker dispatches the coding agent. Board membership is the idempotency marker; a missing initial status is healed, and an existing status is never overwritten.
• banzai-codes/post-candidate-summary (Codex): scans the harness board for Human Review items, picks the oldest unsummarized one, gathers the issue conversation + linked PR (closing-reference lookup, cross-reference fallback) + all image URLs, and has Codex write a non-technical, PM-facing completion summary with the images embedded as proof of work. The summary is posted on the issue with the idempotency marker (<!-- banzai:summary:done -->) embedded inside the summary comment — banzai-codes treats the most recent comment without one of its own chat markers as the candidate summary, so a separate marker comment would shadow it. After posting, the action opens the acceptance gate by moving the Product-board status to Acceptance, which also guarantees the build agent's intermediate comments are never surfaced as a premature candidate. One item per invocation; remaining-count reports the backlog.
• banzai-codes/generate-prd (Codex, manual dispatch): explores the checked-out app and generates/incrementally updates a PRD as a folder of markdown files (docs/prd/: index.md overview + one file per feature area, no file paths or code snippets so it survives refactors). A scope guard fails the run if anything outside the PRD folder changed; the result is force-pushed to a fixed banzai/prd-update branch and opened/updated as a single PR. Used to ground Banzai Codes' define flow in current app behavior.

Since GitHub Actions cannot trigger on Projects v2 status changes, the two scanners are one-shot idempotent scans meant for schedule: cron + workflow_dispatch: wiring — banzai-codes/README.md includes a complete consumer pipeline workflow, token requirements (GitHub App installation token with org Projects RW + Issues RW; default github.token won't work for board scans) and the status-name contract shared with banzai-codes and banzai-codes-worker.

Codex invocation mirrors doright's reusable workflows (openai/codex-action@v1, gpt-5.4, prompt built via github-script, output from final-message).

Motivation and Context

Closes the loop between the Banzai Codes Refinement Studio and the harness execution pipeline: published work orders flow onto the harness board automatically, completed builds flow back as PM-readable candidate summaries at the review gate, and the define flow can be grounded in a maintained PRD of the existing app.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

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