List all functions called by a specific source function. Shows what code a function depends on.

Use this tool when:

• Tracing execution flow to understand what code runs when a function executes
• Identifying dependencies before refactoring
• Understanding function behavior by examining what it calls

Examples:

• See what User.update/2 calls internally to understand its side effects
• Trace database queries triggered by a controller action
• Identify which helper functions are used by a complex operation

Returns: Comprehensive list of called functions with module, name, arity, and file location.