feat: Add GitHub template support for stack deployments

[martin-helmich]

Summary

Adds support for deploying stacks directly from GitHub template repositories using the new --from-template flag.

This enables users to quickly deploy pre-configured application stacks without manually managing docker-compose files locally. Template names like mittwald/n8n automatically resolve to the corresponding mittwald/stack-template-n8n repository.

What This Enables

Quick Stack Deployment
Deploy popular applications instantly from curated templates:

mw stack deploy --from-template=mittwald/n8n --stack-id=s-abc123

Template Repository Naming
Template names follow a simple convention:

• Input: mittwald/n8n
• Resolves to: github.com/mittwald/stack-template-n8n

Environment Configuration
Templates can include default environment variables via .env files, which can be overridden:

# Use template defaults
mw stack deploy --from-template=mittwald/n8n

# Override with local environment
mw stack deploy --from-template=mittwald/n8n --env-file=./production.env

Environment Variable Priority (from lowest to highest):

1. Template .env file (if present)
2. System environment variables
3. Local --env-file flag

Behavior

• Fetches docker-compose.yml and optional .env from the template repository's main branch
• Mutually exclusive with --compose-file flag (prevents accidental mixing of local and remote sources)
• Clear error messages for common issues (template not found, rate limits, network errors)
• No local caching - always fetches fresh template content

Use Cases

• Quick prototyping: Spin up applications without setup overhead
• Standardized deployments: Teams can share vetted stack configurations via templates
• Easy onboarding: New users can deploy complex stacks with a single command
• Version consistency: Templates ensure everyone uses the same configuration

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