fix(#3129): handle trailing json format for diff errors (#3161)

Keep malformed diff invocations with trailing JSON format flags on the parser error path and lock the contract with focused output-format regressions.

Constraint: Do not touch tracked .omx state files.

Rejected: Repeating direct binary smoke loops | local auth/provider configuration intercepts those invocations and obscures parser behavior.

Confidence: high

Scope-risk: narrow

Tested: git diff --check; cargo fmt --check; cargo test -p rusty-claude-cli diff_extra_args_have_typed_error_kind_and_hint_766 --test output_format_contract; cargo test -p rusty-claude-cli diff_trailing_json_after_malformed_args_is_bounded_json_3129 --test output_format_contract; cargo test -p rusty-claude-cli diff_non_git_dir_has_error_kind_and_hint_801 --test output_format_contract

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

@@ -1225,10 +1225,10 @@ fn parse_args(args: &[String]) -> Result<CliAction, String> {
         // `git diff`). No session needed to inspect the working tree.
         "diff" => {
             if rest.len() > 1 {
-                return Err(format!(
-                    "unexpected extra arguments after `claw diff`: {}\nUsage: claw diff",
-                    rest[1..].join(" ")
-                ));
+                // #3129: keep malformed `diff ... --output-format json` on the
+                // parser/error path, not the prompt/TUI fallback. The newline
+                // before Usage is part of the JSON hint contract.
+                return Err(unexpected_diff_args_error(&rest[1..]));
             }
             Ok(CliAction::Diff { output_format })
         }
@@ -1625,6 +1625,13 @@ fn removed_auth_surface_error(command_name: &str) -> String {
     )
 }
 
+fn unexpected_diff_args_error(extra: &[String]) -> String {
+    format!(
+        "unexpected extra arguments after `claw diff`: {}\nUsage: claw diff",
+        extra.join(" ")
+    )
+}
+
 fn parse_acp_args(args: &[String], output_format: CliOutputFormat) -> Result<CliAction, String> {
     match args {
         [] => Ok(CliAction::Acp { output_format }),