Add ShellJS compatibility layer with streaming and virtual command advantages

[konard]

Goal: Beat ShellJS's 9M weekly downloads - our closest competitor with built-in commands

ShellJS Advantages:

• 20+ built-in commands vs our 18
• Mature ecosystem and documentation
• Template literal support in newer versions

Our Advantages to Leverage:

• ✅ Template literals ($cmd``) vs function calls (shell.ls())
• ✅ Real-time streaming (vs buffered only)
• ✅ Virtual commands engine (revolutionary vs none)
• ✅ Mixed pipelines (vs basic piping)
• ✅ EventEmitter pattern (vs none)
• ✅ Async iteration (vs none)
• ✅ Advanced signal handling (vs basic)
• ✅ Public domain license (vs BSD-3-Clause)

Implementation:

• Add 2+ more built-in commands to match ShellJS (22 total)
• Add $.shelljs compatibility mode
• Create migration examples for all ShellJS commands
• Document streaming advantages over ShellJS buffering
• Show virtual commands solving ShellJS extension limitations
• Performance comparisons (streaming vs buffering)

Target Commands to Add:

• tail command (streaming advantage obvious)
• head command
• sort command
• uniq command

Success Metrics:

• Feature parity + streaming advantage
• Complete migration documentation
• Performance benchmarks