fix(harper-cli): return exit code 0 when no lints are found

The `lint()` function unconditionally called `process::exit(1)`,
causing `harper-cli lint` to always exit with code 1 even when no
lints were found. This was a regression introduced in PR #2138.

Now exits with code 1 only when lints are present, and returns
Ok(()) (exit code 0) otherwise.

Fixes #2832

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Ryujiyasu

harper-cli/src/lint.rs

@@ -268,6 +268,8 @@ pub fn lint(
         }
     }
 
+    let has_lints = !all_lint_kinds.is_empty();
+
     final_report(
         dialect,
         true,
@@ -278,7 +280,11 @@ pub fn lint(
         lint_options.color,
     );
 
-    process::exit(1);
+    if has_lints {
+        process::exit(1);
+    }
+
+    Ok(())
 }
 
 type LintKindCount = HashMap<LintKind, usize>;