cargo nextest rewrite rule missing from registry despite subcommand existing

Bug

rtk cargo nextest exists as a valid subcommand with its own filtered output, but rtk rewrite "cargo nextest run" returns exit code 1 (no RTK equivalent), so the Claude Code hook never rewrites cargo nextest calls to use RTK.

Steps to reproduce

# Subcommand exists:
rtk cargo nextest --help   # works fine

# But rewrite rule is missing:
rtk rewrite "cargo nextest run"
echo $?  # → 1

Expected behavior

rtk rewrite "cargo nextest run" should return rtk cargo nextest run (exit 0), so the Claude Code PreToolUse hook transparently rewrites cargo nextest invocations for token savings.

Environment

• rtk 0.40.0
• Hook: rtk hook claude / rtk-rewrite.sh (hook-version 3)

Notes

The fix is likely a missing entry in src/discover/registry.rs for the cargo nextest rewrite rule — the subcommand implementation already exists, only the rewrite registration is absent.