docs(rules): grep -r on a single file omits the filename on GNU, not BSD

[laurigates]

What

Adds a grep row and a subsection to shell-scripting.md's existing GNU vs BSD Tool Differences section.

grep -r on a single file argument prints line:text on GNU and file:line:text on BSD. A guard that greps a file list and parses FILE:LINE:TEXT therefore works on macOS and silently mis-parses on Linux whenever the list happens to hold one entry — the parse shifts a field and reads the line number as the filename.

grep -rn  -F "$pattern" "${files[@]}"   # GNU, one file: "2:gh api ..."
grep -rHn -F "$pattern" "${files[@]}"   # both:          "./path/run.sh:2:gh api ..."

Distilled from a live break, not a hypothetical

scripts/check-dead-api-endpoints.sh (#2321) was 17/17 green on macOS and failed one assertion on its first Linux CI run, reporting FILE=2.

The part worth writing down is that a fixture cannot catch this class — the fixture runs on whichever grep the author has, so the local suite is green by construction. Running on both platforms is the only detector. That is also why it is stated as an unconditional rule rather than a "when you have multiple files" caveat: "expect one file" is doing load-bearing work in the buggy version, and a single-entry list is exactly the trigger.

It sits beside the two BSD date traps because it is the same shape — an implementation-dependent default where the portable form costs one flag.

Cost

None on the always-loaded surface. shell-scripting.md is path-scoped to **/*.sh and scripts/**, so it loads only when shell is in play. check-context-engineering.py --strict still exits 0.

modified: / reviewed: bumped, per the frontmatter convention this repo's own check-docs-index.sh Check 5 enforces.