Add file I/O to standard library

[dev-dami]

Description

Need file system operations in the standard library.

Suggested Functions

• readFile(path) - Read file contents as string
• writeFile(path, content) - Write string to file
• appendFile(path, content) - Append to file
• fileExists(path) - Check if file exists
• deleteFile(path) - Delete a file
• listDir(path) - List directory contents

Considerations

• Return Result types for error handling: readFile("foo.txt")?
• Security: sandboxing? path validation?

Where to Start

• pkg/eval/builtins.go - Add builtin implementations
• Use Go's os and io/ioutil packages

Priority

High - needed for package manager and self-hosting.