Add structured output formatter for setup and agent mode

[DylanBlakemore]

Summary

• Add internal/output package with a Formatter struct for structured, visually clear CLI output
• Support both fancy (box-drawing + unicode) and plain text modes with automatic TTY detection
• Mark section 2.1 complete in the agent setup improvements plan

Details

This introduces the output.Formatter that will replace raw fmt.Printf calls throughout setup execution and agent mode. The formatter provides a consistent visual language for phase boundaries (PhaseStart/PhaseEnd), step results (StepSuccess/StepFailure/StepSkipped), and skipped phase summaries (PhasesSkipped).

In TTY environments, output uses box-drawing characters and ANSI colors for readability. In non-TTY environments (CI, piped output) or when NO_COLOR is set, it falls back to plain text markers like [ok], [FAIL], and [skip]. Both behaviors can be overridden via WithColor() and WithFancy() options.

Tests cover all formatter methods, both output modes, color/no-color, TTY detection, and a full phase sequence integration test (95.3% coverage).