Fix multi-line string escaping issues with special characters

[konard]

Summary

Fixes #37: Multi-line strings containing special shell characters (backticks `, dollar signs $, quotes) are now properly preserved when used with echo commands, preventing corruption due to shell interpretation.

Problem

Previously, complex multi-line strings would get corrupted when passed through command-stream's template literals:

// ❌ Before: Content would be corrupted
const content = `# README
Commands: \`ls -la\`  
Variables: $HOME, $100
`;
await $`echo "${content}" > README.md`; // Content corrupted!

Solution

Automatic Detection & Virtual Command Bypass

• Enhanced quote() function detects multi-line strings with shell special characters
• Complex cases are automatically converted to use a virtual command bypass
• Content is base64-encoded to prevent any shell interpretation
• Simple strings continue to work unchanged (backward compatible)

// ✅ After: Content preserved exactly
const content = `# README
Commands: \`ls -la\`
Variables: $HOME, $100
`;
await $`echo "${content}" > README.md`; // Works perfectly!

Technical Changes

• Enhanced regex patterns: Updated to handle multi-line strings with s flag
• Smart content detection: Automatically detects complex multi-line strings
• Virtual command bypass: _write_multiline_content virtual command for safe file writing
• Base64 encoding: Prevents shell interpretation while preserving exact content

Test Coverage

• ✅ Multi-line strings with backticks, dollar signs, quotes
• ✅ README files, configuration files, code examples
• ✅ Mixed special character combinations
• ✅ Backward compatibility with simple strings
• ✅ Edge cases: empty strings, single-line complex strings

Files Changed

• src/$.mjs: Core logic enhancement
• src/commands/$._write_multiline_content.mjs: New virtual command
• tests/multiline-escaping.test.mjs: Comprehensive test suite
• examples/test-multiline-escaping.mjs: Demonstration script
• docs/MULTILINE_ESCAPING_FIX.md: Technical documentation

Backward Compatibility

✅ All existing functionality preserved. The fix only activates for:

• Multi-line strings (containing \n)
• With shell special characters (backticks ` or dollar signs $)
• In echo ... > file patterns

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