test(output_format_contract): add plugins json coverage to inventory_commands test

Add four assertions to inventory_commands_emit_structured_json_when_requested:
- kind == "plugin"
- action == "list"
- reload_runtime is boolean
- target is null when no plugin is targeted

Closes the only major --output-format json surface with zero contract
coverage. All other surfaces (agents, mcp, skills, status, sandbox,
doctor, help, version, acp, bootstrap-plan, system-prompt, init, diff,
config) already had test assertions.

rust/crates/rusty-claude-cli/tests/output_format_contract.rs

@@ -105,6 +105,18 @@ fn inventory_commands_emit_structured_json_when_requested() {
     let skills = assert_json_command(&root, &["--output-format", "json", "skills"]);
     assert_eq!(skills["kind"], "skills");
     assert_eq!(skills["action"], "list");
+
+    let plugins = assert_json_command(&root, &["--output-format", "json", "plugins"]);
+    assert_eq!(plugins["kind"], "plugin");
+    assert_eq!(plugins["action"], "list");
+    assert!(
+        plugins["reload_runtime"].is_boolean(),
+        "plugins reload_runtime should be a boolean"
+    );
+    assert!(
+        plugins["target"].is_null(),
+        "plugins target should be null when no plugin is targeted"
+    );
 }
 
 #[test]