refactor: extract K8s and Containerfile templates to embedded files

Summary

The K8s manifest templates (ConfigMap, Deployment, Service, ServiceAccount, Secret) and the Containerfile template are currently hardcoded as Go template strings in pkg/manifest/k8s.go and pkg/manifest/containerfile.go.

These should be extracted to separate template files and loaded via go:embed for:

• Readability — templates are easier to edit as standalone files with proper syntax highlighting
• Maintainability — changes to YAML structure don't require modifying Go code
• Customization — users could override templates by providing their own (future)

Files to change

Use //go:embed templates/*.tmpl to load them at compile time.

Context

From PR #74 code review. Deferred to keep the PR focused on the agent manifest feature.

Start by reading the inline templates in pkg/manifest/k8s.go and pkg/manifest/containerfile.go, then review how the manifest package currently renders them. Extract the named Kubernetes and Containerfile templates into pkg/manifest/templates/ and load them with go:embed; done means the package renders the same templates from those files.