fix(newos4): piped grep searched the working directory instead of its input

The stdin work in 5cf5010 fixed wc/head/tail but left grep alone, and grep's no-operand branch
sweeps the cwd. Measured on v4.6.115: `cat /gdir/in.txt | grep zzz` answered
`other.txt:1: zzz in a cwd file` — a file that was never piped, and `cat in.txt | grep bbb`
reported the match twice (`in.txt` from the cwd sweep plus `__pipebuf__` from the injected file).

- module-fs.js: when terminal._pipeInput is set and no file operand was given, grep searches the
  staged text and prints `<line>: <text>` (stdin has no filename, as in GNU). The cwd sweep stays
  for a bare interactive `grep x`.
- shell.js: _isPipeAware now covers only sort/cut/uniq/expand. grep/head/tail/wc read _pipeInput
  themselves, so injecting the __pipebuf__ file produced duplicate hits and named stdin results.

Verification:
- test-shell-stdin.js 20 asserts (+4 for grep: no cwd match, non-match reports none, match prints
  no filename, -c counts stdin). RED on HEAD: G1/G2/G3 fail with `other.txt:1: zzz in a cwd file`.
- Suite 18/18 files; maintain --fix "node tests 18/18 pass".
- Live v4.6.116 after `cd /gdir` (where other.txt contains zzz):
  `cat /gdir/in.txt | grep zzz` -> (no matches) · `| grep bbb` -> `2: bbb` · `| grep -c b` -> 1 ·
  `| head -n 2` -> aaa,bbb · `| tail -n 1` -> ccc · `| wc -l` -> 3.

Credit: the false-positive grep was reported by the stdin subagent as an aside; re-derived here
against the live terminal before fixing.

1 files changed, 16 insertions, 0 deletions

diff --git a/modules/core/core-bundle.js b/modules/core/core-bundle.js
index 6f00658..1a92ef9 100644
--- a/modules/core/core-bundle.js
+++ b/modules/core/core-bundle.js
@@ -6828,6 +6828,22 @@ class FileSystemModule {
                     else this.terminal.print(grand + ' match(es)' + (multi ? ' in ' + readFiles + ' file(s)' : ''), 'text-muted');
                 }
             } else {
+                // Piped input wins over the cwd sweep. `cat f | grep x` used to grep the working
+                // directory, so it could match a file that was never piped — measured 2026-10-27:
+                // `cat /gdir/in.txt | grep zzz` answered "other.txt:1: zzz in a cwd file".
+                const piped = this.terminal._pipeInput;
+                if (piped !== null && piped !== undefined) {
+                    let pipedTotal = 0;
+                    String(piped).split('\n').forEach((line, i) => {
+                        if (!hits(line)) return;
+                        pipedTotal++;
+                        if (!opt.count && !opt.files) this.terminal.print((i + 1) + ': ' + line);
+                    });
+                    if (opt.files) { if (pipedTotal) this.terminal.print('(stdin)'); }
+                    else if (opt.count) this.terminal.print(String(pipedTotal));
+                    else if (pipedTotal === 0) this.terminal.print('(no matches)', 'info');
+                    return;
+                }
                 const dir = this.terminal.fs.getCurrentDir();
                 if (!dir) return this.terminal.print('grep: cannot search', 'error');
                 let total = 0;