fix(mcp): stop surfacing regex-fallback warning alongside matches

When a grep pattern like 'foo(' triggers regex compilation and fails,
the core engine already falls back to literal matching and returns the
correct results. Prepending '! regex failed: ...' on top of those
results only confuses LLM callers (issue #425) — they see the error
before the matches and conclude the call failed.

Drop the warning from the formatter output entirely; the core still
populates the field for debugging and other bindings.

crates/fff-mcp/src/output.rs

@@ -154,7 +154,6 @@ pub struct GrepFormatter<'a> {
     pub files: &'a [&'a FileItem],
     pub total_matched: usize,
     pub next_file_offset: usize,
-    pub regex_fallback_error: Option<&'a str>,
     pub output_mode: OutputMode,
     pub max_results: usize,
     pub show_context: bool,
@@ -169,7 +168,6 @@ impl GrepFormatter<'_> {
             files,
             total_matched,
             next_file_offset,
-            regex_fallback_error,
             output_mode,
             max_results,
             show_context,
@@ -216,10 +214,6 @@ impl GrepFormatter<'_> {
             2500
         };
 
-        if let Some(err) = regex_fallback_error {
-            lines.push(format!("! regex failed: {}, using literal match", err));
-        }
-
         // File overview: collect first match per file
         let file_preview = collect_file_preview(items, files, picker);
         let mut content_def_file = String::new();

crates/fff-mcp/src/server.rs

@@ -273,7 +273,6 @@ impl FffServer {
                             files: &retry_result.files,
                             total_matched: retry_result.matches.len(),
                             next_file_offset: retry_result.next_file_offset,
-                            regex_fallback_error: retry_result.regex_fallback_error.as_deref(),
                             output_mode,
                             max_results,
                             show_context: ctx_lines > 0,
@@ -363,7 +362,6 @@ impl FffServer {
             files: &result.files,
             total_matched: result.matches.len(),
             next_file_offset: result.next_file_offset,
-            regex_fallback_error: result.regex_fallback_error.as_deref(),
             output_mode,
             max_results,
             show_context: ctx_lines > 0,
@@ -604,7 +602,6 @@ impl FffServer {
                         files: &fb_file_refs,
                         total_matched: fb_result.matches.len(),
                         next_file_offset: fb_result.next_file_offset,
-                        regex_fallback_error: None,
                         output_mode,
                         max_results,
                         show_context: false,
@@ -636,7 +633,6 @@ impl FffServer {
             files: &file_refs,
             total_matched: result.matches.len(),
             next_file_offset: result.next_file_offset,
-            regex_fallback_error: None,
             output_mode,
             max_results,
             show_context: ctx_lines > 0,