fix(glob,grep): keep exit-code gate at >1 — --no-messages alone is enough

Addresses cubic-dev-ai P1 + P2 findings on #4115.

The original PR relaxed `exitCode > 1` to `> 2` based on the (wrong)
claim that ripgrep exits 2 only on non-fatal I/O issues. ripgrep
actually uses exit code 2 for BOTH fatal errors (pattern syntax,
invalid args) AND non-fatal I/O issues; GNU grep (the fallback backend
in grep/cli.ts) likewise uses 2 for fatal errors. So `> 2` would
silently suppress fatal errors.

The correct fix is just `--no-messages`, which suppresses ripgrep's
stderr only for soft I/O issues (broken symlinks, permission denied)
while leaving fatal-error messages intact. With the gate kept at
`exitCode > 1 && stderr.trim()`:

- Broken symlink: ripgrep exits 2, stderr is empty (suppressed) →
  `stderr.trim()` is falsy → gate fails → partial results survive.
- Fatal error: ripgrep exits 2, stderr has the real error message
  (not suppressed by --no-messages) → gate triggers → error returned.

Reverting both `exitCode > 1` → `> 2` changes; keeping the
`--no-messages` flag additions and the regression test (test comment
updated to describe the cleaner architecture).

Verification: bun test src/tools/glob/ src/tools/grep/ → 30 pass / 0 fail.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/tools/grep/cli.ts

@@ -197,10 +197,7 @@ async function runRgInternal(options: GrepOptions, resolvedCli?: ResolvedCli): P
     const truncated = stdout.length >= DEFAULT_MAX_OUTPUT_BYTES
     const outputToProcess = truncated ? stdout.substring(0, DEFAULT_MAX_OUTPUT_BYTES) : stdout
 
-    // ripgrep exits 2 on non-fatal I/O errors (broken symlinks, permission
-    // denied) even when stdout still contains valid matches; treat only
-    // exit codes >2 as fatal so partial results survive. See #3726.
-    if (exitCode > 2 && stderr.trim()) {
+    if (exitCode > 1 && stderr.trim()) {
       return {
         matches: [],
         totalMatches: 0,