feat(policy): restrict available models to sonnet, opus, haiku

Add /etc/claude-code/managed-settings.json with availableModels set to
the three Anthropic model families. The file is root-owned inside the
container so the node user cannot modify it. Managed settings cannot be
bypassed via --model flag, /model command, or ANTHROPIC_MODEL env var.

claude/managed-settings.json

@@ -0,0 +1,3 @@
+{
+  "availableModels": ["sonnet", "opus", "haiku"]
+}