Apply structured output formatter to setup execution

[DylanBlakemore]

Summary

• Replace raw fmt.Printf calls in Executor.Execute with the structured output.Formatter
• Add phase lifecycle tracking, per-step durations, stderr on failure, and skipped phase reporting
• Update all executor and runner tests to match new output format

Details

This wires up the output.Formatter (created in #3) into the setup executor. Instead of flat [phase] step lines and emoji-based status messages, the executor now emits structured phase blocks with PhaseStart/PhaseEnd boundaries, shows step durations inline via StepSuccess/StepFailure, displays failing command stderr indented beneath the step, and lists skipped phases at the bottom when execution fails early. A new NewExecutorWithFormatter constructor supports dependency injection for testing. Tests now use a buffer-backed plain-text formatter for deterministic assertions, and two new test cases cover stderr display and skipped phase output.