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/agent/testhelpers_test.go

@@ -22,6 +22,10 @@ func (f *fakeRuntime) Pull(_ context.Context, _ string) error               { re
 func (f *fakeRuntime) Run(_ context.Context, _ runtime.ContainerSpec) error { return nil }
 func (f *fakeRuntime) Stop(_ context.Context, _ string) error               { return nil }
 func (f *fakeRuntime) Remove(_ context.Context, _ string) error             { return nil }
+func (f *fakeRuntime) Build(_ context.Context, _, _, _ string) error        { return nil }
+func (f *fakeRuntime) Push(_ context.Context, _ string) error               { return nil }
+
+func (f *fakeRuntime) ImageExists(_ context.Context, _ string) (bool, error) { return true, nil }
 
 func (f *fakeRuntime) List(_ context.Context) ([]runtime.ContainerInfo, error) {
 	return f.containers, f.listErr