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/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"