fix(pnpm): allow flag-first args to pass through

[TechWizard9999]

Description

This fixes rtk pnpm argument parsing for flag-first invocations so RTK no longer fails before passthrough can run.
Commands like rtk pnpm --filter mymodule test are now routed to native pnpm safely.

Related Issue

Closes #259

Changes Made

• Reworked rtk pnpm CLI parsing to accept raw pnpm args (instead of strict clap subcommands)
• Added runtime routing for known optimized paths (list, outdated, install, typecheck)
• Added explicit passthrough for flag-first pnpm forms and unsupported subcommands
• Added regression tests for pnpm flag-first parsing and list-depth extraction
• Updated changelog unreleased bug fixes

Testing

• cargo fmt --all --check
• rtk cargo clippy --all-targets
• rtk cargo test --all
• Manual reproduction:
  • rtk cargo run -- pnpm --filter mymodule test --help
  • Result now reaches pnpm (no RTK clap usage error)

@aeppling please review when you have a moment.