Add mcp build command and deploy auto-build

Extends MCP to own the full build-push-deploy lifecycle. When deploying,
the CLI checks whether each component's image tag exists in the registry
and builds/pushes automatically if missing and build config is present.

- Add Build, Push, ImageExists to runtime.Runtime interface (podman impl)
- Add mcp build <service>[/<image>] command
- Add [build] section to CLI config (workspace path)
- Add path and [build.images] to service definitions
- Wire auto-build into mcp deploy before agent RPC
- Update ARCHITECTURE.md with runtime interface and deploy auto-build docs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/servicedef/servicedef.go

@@ -18,9 +18,17 @@ type ServiceDef struct {
 	Name       string         `toml:"name"`
 	Node       string         `toml:"node"`
 	Active     *bool          `toml:"active,omitempty"`
+	Path       string         `toml:"path,omitempty"`
+	Build      *BuildDef      `toml:"build,omitempty"`
 	Components []ComponentDef `toml:"components"`
 }
 
+// BuildDef describes how to build container images for a service.
+type BuildDef struct {
+	Images   map[string]string `toml:"images"`
+	UsesMCDSL bool            `toml:"uses_mcdsl,omitempty"`
+}
+
 // RouteDef describes a route for a component, used for automatic port
 // allocation and mc-proxy integration.
 type RouteDef struct {