feat(bashkit-cli): Add CLI binary for command line usage

[chaliy]

Summary

Add a CLI binary for running BashKit from the command line.

Usage

# Execute a command string
bashkit -c 'echo hello'

# Execute a script file
bashkit script.sh

# Show help
bashkit --help

Features

• -c <command> - Execute a command string
• Script file execution
• Proper exit code handling
• stdout/stderr separation
• Error messages via anyhow

Examples

# Simple echo
$ bashkit -c 'echo "Hello, World!"'
Hello, World!

# Arithmetic
$ bashkit -c 'echo $((10 + 20))'
30

# Arrays
$ bashkit -c 'arr=(a b c); echo ${#arr[@]}'
3

# For loop
$ bashkit -c 'for i in 1 2 3; do echo $i; done'
1
2
3

Test plan

• 104 tests passing (library)
• CLI help works
• Command execution works
• Script file execution works
• Clippy clean

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