Apply structured output formatter to agent mode

[DylanBlakemore]

Summary

• Add agent-mode formatting methods to the output Formatter (header, round separator, explanation, action list, action results)
• Wire formatter into agent mode in cmd/setup.go, replacing raw fmt.Println calls
• Add 15 tests covering all new formatter methods in plain and fancy modes

Details

This completes section 2.3 of the agent setup improvements plan. When the agent enters diagnosis mode after a setup failure, users now see structured, visually clear output instead of raw text dumps.

In TTY environments, the output uses box-drawing characters and color-coded danger badges ([safe], [caution], [dangerous]). The agent header shows which phase and step failed, each round gets a visual separator, the diagnosis explanation is displayed in an indented quoted block, and proposed actions appear as a numbered list. After execution, results are shown inline with check/cross marks.

Non-TTY environments get a clean plain-text fallback with ===, --, > prefixes, and [ok]/[FAIL] markers — no ANSI codes or box-drawing characters.