Browser shell grep -q returns success for non-matching input

In the browser shell demo, GNU grep currently returns status 0 for matching and non-matching input when run from the PTY shell. This made the merge-gate NetHack check report `KANDELO_NETHACK_BAD` even though `/tmp/kandelo-nethack.out` did not contain `Cannot open record file`.

Focused browser repro from the shell demo:

```sh
printf 'abc\\n' >/tmp/g\ngrep -q zzz /tmp/g; echo "GREP_FILE_NO:$?"\ngrep -q abc /tmp/g; echo "GREP_FILE_YES:$?"\nprintf 'abc\\n' | grep -q zzz; echo "GREP_STDIN_NO:$?"\nprintf 'abc\\n' | grep -q abc; echo "GREP_STDIN_YES:$?"\n```\n\nObserved in Chromium against `apps/browser-demos` on this branch after rebuilding `shell.vfs.zst`:\n\n```text\nGREP_FILE_NO:0\nGREP_FILE_YES:0\nGREP_STDIN_NO:0\nGREP_STDIN_YES:0\n```\n\nExpected: no-match cases should exit 1. The existing package tests cover grep stdin through `runCentralizedProgram`, but the browser PTY path needs coverage for grep exit status and file operands.

Start in apps/browser-demos and reproduce the four grep commands through the browser PTY shell after rebuilding shell.vfs.zst. Compare that path with the existing grep stdin coverage in runCentralizedProgram, then add browser PTY coverage for matching and non-matching stdin and file operands. Done means non-matches exit 1 and matches exit 0.

Written by the indexing model from the issue text.