mcp: prefer ast-grep binary over deprecated sg alias

Newer @ast-grep/cli deprecated the `sg` alias and prints a
"WARNING: `sg` is deprecated" banner into the captured --json output,
which corrupts the parse in cpp_grep_usage / cpp_find_symbol / cpp_outline
(the 21 'Failed to parse ast-grep output: invalid character =' failures in
run_utils_tests). Prefer the canonical `ast-grep` binary (ships in the same
package, emits clean JSON); keep `sg` as a fallback for older installs.

Repo-wide CI issue, present on master too (extended checks #3550).

Author: aleksisch

utils/mcp/tools/grep_usage.das

@@ -30,7 +30,7 @@ def detect_ast_grep() : string {
     // (`npm install -g @ast-grep/cli`) the global PATH only gets `sg.cmd`, so
     // CreateProcess(NULL, "sg ...") fails — fall through to the explicit
     // `.cmd` / `.exe` / `.bat` candidates so the shim is picked up.
-    let candidates = ["sg", "sg.exe", "sg.cmd", "sg.bat"]
+    let candidates = ["ast-grep", "ast-grep.cmd", "sg", "sg.exe", "sg.cmd", "sg.bat"]
     for (c in candidates) {
         if (try_ast_grep(c)) return c
     }