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/podman.go

@@ -3,6 +3,7 @@ package runtime
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"os/exec"
 	"strings"
@@ -177,6 +178,40 @@ func (p *Podman) Inspect(ctx context.Context, name string) (ContainerInfo, error
 	return info, nil
 }
 
+// Build builds a container image from a Dockerfile.
+func (p *Podman) Build(ctx context.Context, image, contextDir, dockerfile string) error {
+	args := []string{"build", "-t", image, "-f", dockerfile, contextDir}
+	cmd := exec.CommandContext(ctx, p.command(), args...) //nolint:gosec // args built programmatically
+	cmd.Dir = contextDir
+	if out, err := cmd.CombinedOutput(); err != nil {
+		return fmt.Errorf("podman build %q: %w: %s", image, err, out)
+	}
+	return nil
+}
+
+// Push pushes a container image to a remote registry.
+func (p *Podman) Push(ctx context.Context, image string) error {
+	cmd := exec.CommandContext(ctx, p.command(), "push", image) //nolint:gosec // args built programmatically
+	if out, err := cmd.CombinedOutput(); err != nil {
+		return fmt.Errorf("podman push %q: %w: %s", image, err, out)
+	}
+	return nil
+}
+
+// ImageExists checks whether an image tag exists in a remote registry.
+func (p *Podman) ImageExists(ctx context.Context, image string) (bool, error) {
+	cmd := exec.CommandContext(ctx, p.command(), "manifest", "inspect", "docker://"+image) //nolint:gosec // args built programmatically
+	if err := cmd.Run(); err != nil {
+		// Exit code 1 means the manifest was not found.
+		var exitErr *exec.ExitError
+		if ok := errors.As(err, &exitErr); ok && exitErr.ExitCode() == 1 {
+			return false, nil
+		}
+		return false, fmt.Errorf("podman manifest inspect %q: %w", image, err)
+	}
+	return true, nil
+}
+
 // podmanPSEntry is a single entry from podman ps --format json.
 type podmanPSEntry struct {
 	Names   []string `json:"Names"`