Two fixes:
1. publish-canvas-image.yml + publish-platform-image.yml: the JSON
heredoc for config.json had leading whitespace from YAML indentation,
producing invalid JSON. Docker fell back to osxkeychain → -25308.
Fixed by removing indentation inside the heredoc body.
2. Added scripts/dev-start.sh — one-command local dev environment.
Starts infra (docker-compose), platform (Go), and canvas (Next.js)
with proper health checks and cleanup on Ctrl-C.
