Auto-parse pasted PM commands in pm add

[fdarian]

Summary

• Add ability to auto-parse pasted package manager commands (npm/pnpm/bun) in pm add
• Users can now paste commands like npm install -D something or pnpm add foo bar directly
• Command parsing intelligently extracts packages and dev flags, merging with CLI flags
• Add comprehensive test suite with 16 test cases covering edge cases and error scenarios

Test plan

• Unit tests pass for passthrough cases (single/multiple packages)
• Unit tests pass for npm command parsing (install, i, with -D and --save-dev)
• Unit tests pass for pnpm command parsing (add, install, with -D)
• Unit tests pass for bun command parsing
• Unit tests pass for shell-split multi-arg scenarios
• Unit tests pass for dev flag merging logic
• Unit tests pass for error scenarios (no packages specified)
• Manual test: pm add npm install -D typescript should add typescript as dev
• Manual test: pm add pnpm add foo bar should add both packages
• Manual test: Error thrown when no packages in command