refactor(interpreter): extract inline builtins from execute_dispatched_command

Extract 6 blocks from the 574-line execute_dispatched_command() into
separate methods: execute_function_call, execute_local_builtin,
execute_trap_builtin, execute_let_builtin, execute_unset_builtin,
execute_caller_builtin. No behavioral changes.

Closes #313

https://claude.ai/code/session_01WZjYqxm5xMPAEe7FSHJkDy

Author: claude