ux: improve empty state when prime-agent binary is missing

[mcpeezy]

Why

First-run failure mode is the #1 stranger friction.

Scope

• When doctor/runtime detects missing prime-agent, show a clearer empty-state with:
  1. Install command
  2. Repair menu path
  3. Link/button to open install docs if already present
• Prefer copy + existing menu action wiring over new architecture

Hints

• scripts/doctor.js
• renderer empty-state components under renderer/
• docs/INSTALL.md

Acceptance

• Missing-agent state is obvious in one screen
• npm test and npm run smoke pass
• No voice features

Out of scope

Auto-downloading binaries without user consent; Windows/Linux ports.

[epiphany-github-guardian]

Status: 🆕 status:triage
Priority: 🟡 priority:P2
Evidence: Guardian recorded 0 changed file(s) and a reported combined check state from supplied GitHub evidence.
Next action: Review this pull request and update its disposition.
Owner: Repository maintainer