fix(grep): respect the invoked tool — grep runs grep, rg runs ripgrep

[olsavmic]

Summary

rg and grep were both rewritten to rtk grep, which always ran ripgrep as its backend. That collapsed the two tools into one and broke whichever didn't match the backend:

• ripgrep-only flags (--glob, -g, -P, --files) failed — the command fell through to system grep, which rejects them (rtk grep calls system grep instead of ripgrep — breaks -g/--glob flags #2167, rg --files is rewritten to rtk grep --files #2060, rtk fails to handle --pcre2 flag when agent tries to use rg #1651);
• grep commands silently misbehaved under rg — e.g. -r is recursive in grep but --replace in rg, so grep -rn foo rewrote every match to n instead of searching.

This PR preserves the invoked tool's identity from the rewrite onward, so each command runs its own backend with its own semantics — no crossover in either direction.

Design

Invoked	Rewritten to	Runs	Semantics
grep …	rtk grep	system grep	grep (BRE, grep flags, -r)
rg …	rtk rg	ripgrep	rg (--glob/-g/-P/--files, any order)

• rewrite: split the single ^(rg\|grep) rule into grep → rtk grep and rg → rtk rg.
• rtk rg (new subcommand): faithful ripgrep passthrough — all args forwarded verbatim (flat trailing_var_arg, so leading flags never break clap) — with RTK's by-file filtering; info/list flags (--files, --json, …) stream raw.
• rtk grep: now runs system grep, parsing grep's file:line:content output; -t/--file-type maps to grep --include. Keeps its documented -l/-m/-t/--context-only interface.
• shared emit_filtered takes a per-backend line parser (rg NUL form file\0line:content vs grep colon form).

Why not "route everything through one backend"

The first iterations of this PR tried that (route rg-style fallbacks to rg). Review found it corrupts the other tool because grep and rg share short flags with conflicting meanings — most dangerously grep -rn foo → rg --replace=n → every match rewritten. There is no safe way to run one tool's syntax through the other; the only correct fix is to keep them separate.

Test plan

• cargo fmt --check && cargo clippy --all-targets && cargo test — 1985 passed, 7 ignored, clippy clean, fmt clean
• Unit tests: rewrite split (rg→rtk rg, grep→rtk grep), parse_grep_line, grep_type_glob, is_raw_passthrough_flag
• Manual verification (built binary):

# rewrite preserves origin
rg --glob '*.py' alpha .   ->  rtk rg --glob '*.py' alpha .
grep -rn alpha .           ->  rtk grep -rn alpha .

# rtk rg: ripgrep, faithful + filtered
$ rtk rg --glob '*.py' alpha gp     # was: grep: unrecognized option `--glob'
2 matches in 2 files: ...
$ rtk rg -P 'alpha\s' gp/a.py        # was: grep: invalid option -- P
1 matches in 1 files: ...
$ rtk rg --files gp                  # issue #2060

# rtk grep: system grep, correct (no rg --replace corruption)
$ rtk grep -rn alpha gp              # earlier naive routing gave "n one"
gp/a.py:1:alpha one ...
$ rtk grep -E 'alpha|beta' gp/a.py   # grep ERE, respected

Notes

• rtk grep now runs system grep rather than ripgrep — a deliberate behavior change so grep semantics are respected (loses rg's speed/gitignore-awareness for grep commands; that's grep being grep). rtk rg is the path for ripgrep.
• Verified on macOS (BSD grep) + ripgrep 15.1.0; logic is platform-agnostic but not yet exercised on GNU grep.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}