refactor(setup): drop Apple Container support

Apple Container is no longer supported — the runtime abstraction in
src/container-runtime.ts is already Docker-only. Remove the remaining
setup-time branches that probed for it: the Apple Container runtime
option in the container build step, the APPLE_CONTAINER field emitted
by the environment check, and the `command -v container` probe in
verify. `--runtime docker` still parses for backwards compatibility
with the /setup skill.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

setup/verify.ts

@@ -85,15 +85,10 @@ export async function run(_args: string[]): Promise<void> {
   // 2. Check container runtime
   let containerRuntime = 'none';
   try {
-    execSync('command -v container', { stdio: 'ignore' });
-    containerRuntime = 'apple-container';
+    execSync('docker info', { stdio: 'ignore' });
+    containerRuntime = 'docker';
   } catch {
-    try {
-      execSync('docker info', { stdio: 'ignore' });
-      containerRuntime = 'docker';
-    } catch {
-      // No runtime
-    }
+    // Docker not running
   }
 
   // 3. Check credentials