feat(builtins): add rg (ripgrep) command

[chaliy]

Context

Parity with vercel-labs/just-bash. rg is a popular fast grep alternative, commonly used by AI agents.

Requirements

• rg PATTERN — recursive grep from cwd
• rg PATTERN path — search in specific path
• rg -i — case insensitive
• rg -n — line numbers (default on)
• rg -l — files with matches only
• rg -c — count matches
• rg --type / rg -t — filter by file type
• rg --glob / rg -g — glob filter
• Can delegate to existing grep -r internally