fix: support /plugins slash command in resume mode

Move SlashCommand::Plugins out of the 'unsupported resumed slash
command' catch-all and add a handler arm in run_resume_command that
calls handle_plugins_slash_command for list/help actions.

Mutation actions (install/uninstall/enable/disable) are rejected with
a clear error since there is no runtime to reload in resume mode.

Add /plugins coverage to resumed_inventory_commands test in
output_format_contract.rs: kind, action, reload_runtime, target.

Before: claw --resume session.jsonl /plugins --output-format json
-> {error: 'unsupported resumed slash command', type: 'error'}, exit 1

After: claw --resume session.jsonl /plugins --output-format json
-> {kind: 'plugin', action: 'list', ...}, exit 0

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

@@ -397,6 +397,35 @@ fn resumed_inventory_commands_emit_structured_json_when_requested() {
         agents["count"].is_number(),
         "count must be a number, not a text render"
     );
+
+
+    let plugins = assert_json_command_with_env(
+        &root,
+        &[
+            "--output-format",
+            "json",
+            "--resume",
+            session_path.to_str().expect("utf8 session path"),
+            "/plugins",
+        ],
+        &[
+            (
+                "CLAW_CONFIG_HOME",
+                config_home.to_str().expect("utf8 config home"),
+            ),
+            ("HOME", home.to_str().expect("utf8 home")),
+        ],
+    );
+    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]