fix(ui): coach summary owns the top card and no longer duplicates on Today - #42

Summary

On the Today tab the coach summary rendered twice: the top HeroInsightCardView
and a bottom CoachMessageCard both derive from TodayInsights.deriveHero, so
whenever the coach summary fell back to the hero text (coach off, or the summary
call failed) both cards showed the same "Steady build" content.

• Today: the coach now owns the single top card (tap → chat) when enabled;
  the deterministic hero fills that slot only when the coach is off. The
  duplicate bottom card is removed.
• Sleep: the coach card moves to the top, in line with Today.

Related issues

N/A

Type of change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• 🎨 UI / DesignSystem change

How was this tested?

• Ran the test suite locally (244 tests, 0 failures)
• Tested with demo data (no hardware) — sideloaded build: Today shows one coach card (no duplicate); Sleep coach card renders at the top
• Tested on a physical device with a real ring

Privacy & data

• This change does not send health data off-device without explicit user action.
• No secrets, API keys, or personal data are committed.

Checklist

• My code follows the project's style (SwiftLint passes — no new violations).
• I ran the tests and they pass.