feat(grep): add -L (--files-without-match) flag

[chaliy]

Summary

The grep builtin is missing the -L / --files-without-match flag, which prints names of files that do NOT contain any matches. This is the inverse of the existing -l flag.

Why we need this

• GNU grep compatibility: -L is a standard GNU grep flag used in everyday shell scripts and CI pipelines (e.g., grep -rL 'TODO' src/ to find files without TODOs).
• LLM tool usage: Coding agents frequently use grep -rL to find files that are missing expected content (missing license headers, missing imports, missing config entries). Without -L, agents must work around it with grep -rl | comm or other multi-step alternatives.
• Trivial to implement: We already have -l (files with matches). -L is the exact inverse — print the filename when match_count == 0 instead of > 0.

Current state

The grep builtin (crates/bashkit/src/builtins/grep.rs) supports 27+ flags including -l, -r, -A/-B/-C, --include, --exclude, etc. -L is one of the few standard flags still missing.

Implementation notes

• Add files_without_matches: bool to GrepOptions
• Parse -L alongside -l
• In the output phase: if files_without_matches && !file_matched, print filename
• Add tests for stdin and multi-file cases