fix: empty-prompt error now returns non-null hint via newline-delimited usage string

claw '' and claw '   ' returned empty_prompt + hint:null because the
error message had no newline delimiter. Added usage hint. 61 CLI
contract tests pass.

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

@@ -1378,8 +1378,9 @@ fn parse_args(args: &[String]) -> Result<CliAction, String> {
             // an empty prompt when credentials are present).
             let joined = rest.join(" ");
             if joined.trim().is_empty() {
+                // #798: add \n hint so split_error_hint extracts it (was empty_prompt + null)
                 return Err(
-                    "empty prompt: provide a subcommand (run `claw --help`) or a non-empty prompt string"
+                    "empty prompt: provide a subcommand or a non-empty prompt string.\nUsage: claw <subcommand> or claw -p <prompt>. Run `claw --help` for the full list."
                         .to_string(),
                 );
             }

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

@@ -3418,3 +3418,36 @@ fn plugins_extra_args_have_non_null_hint_797() {
         "hint should reference plugins usage, got: {h:?}"
     );
 }
+
+#[test]
+fn empty_prompt_has_non_null_hint_798() {
+    // #798: `claw --output-format json ""` returned empty_prompt + hint:null.
+    // The error message "empty prompt: provide a subcommand..." had no \n delimiter.
+    let root = unique_temp_dir("empty-prompt-798");
+    fs::create_dir_all(&root).expect("temp dir");
+    std::process::Command::new("git")
+        .args(["init", "-q"])
+        .current_dir(&root)
+        .output()
+        .ok();
+
+    let output = run_claw(&root, &["--output-format", "json", ""], &[]);
+    assert!(
+        !output.status.success(),
+        "empty prompt must exit non-zero (#798)"
+    );
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    let j: serde_json::Value = stderr
+        .lines()
+        .find(|l| l.trim_start().starts_with('{'))
+        .and_then(|l| serde_json::from_str(l).ok())
+        .expect("empty prompt should emit JSON error envelope");
+    assert_eq!(j["error_kind"], "empty_prompt");
+    let h = j["hint"]
+        .as_str()
+        .expect("empty_prompt must have non-null hint (#798)");
+    assert!(
+        h.contains("claw") || h.contains("Usage"),
+        "hint should reference usage, got: {h:?}"
+    );
+}