[M0-S13] Document the production pipeline and the two-agent workflow #91

Problem

Two agents now build in parallel: Alexander's Claude (with the OpenAI API, so it can generate art) and Vatsal's Codex (no OpenAI key). Without one written pipeline and fixed hand-over formats they will redraw the same things, change the same files and wait on each other.

Acceptance criteria

• docs/13 shows, in text and Mermaid, how every image is generated and processed: direction → stage concepts → chunks → layers → collision annotation → game, and hero sheet → sprite boards → atlases → animations
• It lists every sprite sheet, stage chunk and annotation layer with the issue that delivers it
• It fixes the hand-over contracts both agents code against: atlas + animation JSON, stage manifest, collision JSON
• It defines the two lanes (who generates art, who writes game code), branches, stacks and the sync points between them
• AGENTS.md, docs/12 and docs/10 point to it

Notes and dependencies

Suggested owner: Alexander (pipeline, art, scenes, deploy). Blocks nothing; every agent reads it before its next PR.

Verification

python scripts/check_docs.py; the Mermaid diagrams render on GitHub