feat: add 'prepare' skill for Defang compose compatibility

[simple-agent-manager]

Summary

• Adds a new skills/prepare.md skill that guides agents to prepare a project for deployment with Defang
• Handles both creating compose files / Dockerfiles from scratch and adapting existing ones
• Covers the full checklist: unsupported directives, volumes, ports, secrets/config, service discovery, healthchecks, resource reservations, x-defang extensions, build config, and GitHub Actions

Key areas addressed

Based on research across samples, docs, and CLI validation code, the skill specifically tackles common confusion points:

• Service discovery: explains service-name-as-hostname pattern, mode: host requirement for internal DNS
• Build args: empty values skipped with warning, explicit values needed, frontend URL baking
• GitHub Actions config: config-env-vars for secrets, OIDC auth, AWS_PROFILE gotcha
• Volumes: not supported — guides toward managed services and compose.local.yaml with extends

Test plan

• Review skill content for accuracy against current CLI validation
• Test with an agent on a project that has no compose file (creation flow)
• Test with an agent on a project with an existing non-Defang compose file (adaptation flow)

🤖 Generated with Claude Code

[jordanstephens]

I'm not opposed to this, but I suspect we could push at least some of this down a layer into the cli itself—we shouldn't need a language model for many of these.