The grep argument-parsing rework routed many flag shapes through the
grouping reparse, which silently dropped any line it could not parse and
returned a success exit. Common invocations produced wrong or empty
output -- and grep-only syntax like `--include` failed outright -- which
pushed agents back to unfiltered `rtk proxy grep`.
Every valid grep invocation now produces correct output:
- Context (`-A`/`-B`/`-C`) is grouped and compressed instead of dropped;
the header counts matches only and context lines are shown.
- Output the reparse cannot represent (`-N`, `-I`, `--color`, `-p`,
`--heading`, binary-file notices, ...) is passed through verbatim
instead of reported as a false "0 matches".
- Format/shape flags (`-c`, `-l`, `-o`, `-Z`, `--column`, `-b`,
`--vimgrep`, `--null-data`) pass through cleanly with no NUL leak.
- grep-only syntax (`--include`, `--exclude`, ...) works: ripgrep is the
fast path and rtk falls back to system grep when ripgrep rejects a flag.
- rtk grep never emits more than plain grep: if grouping is not smaller
than `file:line:content`, the raw form is shown.
Resolves#2543.
Adrien Eppling