fix(pnpm): rewrite pnpm commands with global flags before the subcommand

The hook rewriter matched pnpm via a subcommand allowlist anchored right
after `pnpm ` (`^pnpm\s+(install|list|...)`), so flag-first monorepo forms
were never rewritten and streamed raw:

  pnpm run build          -> rtk pnpm run build   (matched)
  pnpm -r install         -> (not rewritten)
  pnpm --filter @app list -> (not rewritten)
  pnpm -w install         -> (not rewritten)

Add `strip_pnpm_global_opts` (mirror of the existing `strip_git_global_opts`)
that strips a fixed set of pnpm global options (`-r`/`--recursive`,
`-w`/`--workspace-root`, `--filter`/`-F`) for CLASSIFICATION only. The rewrite
step still operates on the original text via `strip_word_prefix`, so an
unknown flag or a flag-first form with no matching rewrite prefix is a safe
no-op — never a malformed rewrite. `rtk pnpm` now accepts `-r`/`-w` globally
and forwards them.

The flags are forwarded after the subcommand (`pnpm install -r`), matching the
established `--filter` handling. This is behavior-preserving: they are
root-level pnpm options accepted in either position (verified against pnpm
9.15.4; `pnpm <flag> <sub>` and `pnpm <sub> <flag>` produce identical output
and install scope for `-r`, `-w`, and `--filter`).

Adds 16 tests: flag-first rewrite forms, no-regression on bare forms, and
false-positive guards (unknown flag not stripped, filter without subcommand,
recursive-lint safe no-op).