fix(grep): free `-l` for GNU --files-with-matches instead of --max-len

`-l` was bound to RTK's `--max-len`, colliding with GNU grep where `-l`
means `--files-with-matches`. Running `grep -l <pattern>` made clap read
the pattern as a usize and error (`invalid value '<pattern>' for
'--max-len'`), dropping the command to a raw-grep fallback at 0% savings.

Drop the short binding. `-l`/`-L` now flow to `extra_args`, where the
existing `has_format_flag` already routes them to the grep passthrough
(GNU semantics, minimal output). `--max-len` keeps its long form and its
default of 80; nothing internal used the short.

- src/main.rs: remove `short = 'l'` from `Grep::max_len` + clap-parse
  regression test.
- tests/grep_faithful_format_test.rs: end-to-end test asserting
  `rtk grep -l/-L` is byte-identical to `grep -l/-L`, covering `-l`
  leading and trailing.
- scripts/benchmark.sh: the max-len bench used the old `-l 40` alias ->
  `--max-len 40`.
- docs/usage/FEATURES.md: drop the `-l` short from the grep options table.