fix(builtins): make xargs execute commands instead of echoing

[chaliy]

Summary

• Fix xargs to execute commands through the interpreter instead of echoing them as text
• Intercept xargs at the interpreter level (alongside timeout, eval, source) and dispatch constructed SimpleCommands via execute_command()
• Add 5 integration tests covering: basic execution, default echo, newline splitting, -n 1 per-item execution, and -I {} replacement

Closes #346

Test plan

• test_xargs_executes_command - verifies echo file.txt | xargs cat reads file contents
• test_xargs_default_echo - verifies echo 'a b c' | xargs defaults to echo
• test_xargs_splits_newlines - verifies multi-line input is split into separate args
• test_xargs_n1_executes_per_item - verifies -n 1 executes once per argument
• test_xargs_replace_str - verifies -I {} substitution with actual execution
• All existing xargs unit tests still pass (option parsing, -d, -0, etc.)
• cargo fmt --check clean
• cargo clippy -p bashkit --all-targets --all-features -- -D warnings clean
• cargo test -p bashkit --lib --all-features - 1109 tests pass
• cargo test -p bashkit --all-features --tests - 118 integration tests pass