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/runtime/runtime.go

@@ -34,7 +34,9 @@ type ContainerInfo struct {
 	Started time.Time // when the container started (zero if not running)
 }
 
-// Runtime is the container runtime abstraction.
+// Runtime is the container runtime abstraction. The first six methods are
+// used by the agent for container lifecycle. The last three are used by the
+// CLI for building and pushing images.
 type Runtime interface {
 	Pull(ctx context.Context, image string) error
 	Run(ctx context.Context, spec ContainerSpec) error
@@ -42,6 +44,10 @@ type Runtime interface {
 	Remove(ctx context.Context, name string) error
 	Inspect(ctx context.Context, name string) (ContainerInfo, error)
 	List(ctx context.Context) ([]ContainerInfo, error)
+
+	Build(ctx context.Context, image, contextDir, dockerfile string) error
+	Push(ctx context.Context, image string) error
+	ImageExists(ctx context.Context, image string) (bool, error)
 }
 
 // ExtractVersion parses the tag from an image reference.