Hook matcher: leading backslash-newline defeats command rewrite

If a Bash command starts with a backslash-newline (line continuation), rtk hook check reads the literal \ as the leading token and bails out instead of stripping the continuation first. Claude Code emits this shape often when formatting long invocations, so it shows up in real traffic.

Repro on rtk 0.37.2 (brew, macOS):

$ rtk hook check $'\\\ngit diff HEAD~1'
No rewrite for: \
git diff HEAD~1

Without the leading \<newline>, the same command rewrites cleanly to rtk git diff HEAD~1.

rtk discover -a -s 30 shows ~15 calls in the last 30 days that slipped past the hook this way — almost all git, gh, find, i.e. commands with working handlers. Pure matcher-side recoverable savings.

Related: #1243 (heredoc / multi-line) and #1252 ($(…), backticks, xargs) — same family of "matcher tokenizes too literally", but a distinct surface (leading-whitespace normalization, not nested-context detection).