fix: mcp show (missing server name) emits missing_argument error_kind (#830)

`claw --output-format json mcp show` (no server name) previously emitted
error_kind:"unknown_mcp_action" — misleading because `show` IS a known
action; the problem is a missing required argument.

Fix:
- `render_mcp_report_json_for`: `Some("show")` arm now emits a dedicated
  JSON response with error_kind:"missing_argument" + usage hint
- `classify_error_kind`: add classifier arm for "missing_argument:" prefix

One new test: mcp_show_missing_server_name_emits_missing_argument

572+ tests pass.

rust/crates/commands/src/lib.rs

@@ -3027,7 +3027,19 @@ fn render_mcp_report_json_for(
             }
         }
         Some(args) if is_help_arg(args) => Ok(render_mcp_usage_json(None)),
-        Some("show") => Ok(render_mcp_usage_json(Some("show"))),
+        // #830: `claw mcp show` with no server name is a missing required
+        // argument, not an unknown action. Emit a dedicated error_kind so
+        // machine consumers can distinguish "I know show, but need a name"
+        // from "I don't know this action".
+        Some("show") => Ok(serde_json::json!({
+            "kind": "mcp",
+            "action": "show",
+            "status": "error",
+            "ok": false,
+            "error_kind": "missing_argument",
+            "hint": "Usage: claw mcp show <server>\nRun `claw mcp list` to see available servers.",
+            "message": "missing required argument: mcp show requires a server name.",
+        })),
         Some(args) if args.split_whitespace().next() == Some("show") => {
             let mut parts = args.split_whitespace();
             let _ = parts.next();

rust/crates/rusty-claude-cli/src/main.rs

@@ -271,7 +271,9 @@ Run `claw --help` for usage."
 /// matching against the error messages produced throughout the CLI surface.
 fn classify_error_kind(message: &str) -> &'static str {
     // Check specific patterns first (more specific before generic)
-    if message.starts_with("unknown_slash_command:") {
+    if message.starts_with("missing_argument:") || message.starts_with("missing required argument:") {
+        "missing_argument"
+    } else if message.starts_with("unknown_slash_command:") {
         "unknown_slash_command"
     } else if message.starts_with("command_not_found:") {
         "command_not_found"

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

@@ -4054,3 +4054,33 @@ fn resume_safe_interactive_only_hint_includes_resume_suggestion() {
         "/diff hint must suggest --resume (it is resume-safe) (#829): hint={hint:?}"
     );
 }
+
+// #830: claw mcp show (missing server name) must emit missing_argument, not unknown_mcp_action
+#[test]
+fn mcp_show_missing_server_name_emits_missing_argument() {
+    let root = unique_temp_dir("mcp-show-missing-830");
+    std::fs::create_dir_all(&root).expect("create temp dir");
+    let output = run_claw(&root, &["--output-format", "json", "mcp", "show"], &[]);
+    assert_eq!(output.status.code(), Some(1), "mcp show (no name) should exit 1");
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    let j: serde_json::Value =
+        serde_json::from_str(stdout.trim()).expect("mcp show (no name) must emit JSON (#830)");
+    assert_eq!(
+        j["error_kind"], "missing_argument",
+        "mcp show (no name) must emit missing_argument, not unknown_mcp_action (#830): {j}"
+    );
+    assert_ne!(
+        j["error_kind"], "unknown_mcp_action",
+        "mcp show (no name) must not emit unknown_mcp_action (#830): {j}"
+    );
+    let hint = j["hint"].as_str().unwrap_or("");
+    assert!(
+        hint.contains("claw mcp show") || hint.contains("mcp list"),
+        "mcp show (no name) hint should mention usage (#830): {hint:?}"
+    );
+    assert!(
+        stderr.is_empty(),
+        "mcp show (no name) JSON must have empty stderr (#830): {stderr:?}"
+    );
+}