Nav gets Agent Ready, a dialog with the one-line agent prompt

Agent Ready sits after Compare in the bar and in the menu. It is a link to agent.md that opens a native dialog instead. If the dialog code fails to load, the link opens agent.md as a page.

• The Quickstart tab holds the prompt, a task picker (test a docs page, audit a repo, check a receipt), the 6 steps from agent.md with a copy button each, a macOS/Linux and Windows switch, two real JSON excerpts, the safety chips and the agent links.
• The Full agent.md tab fetches agent.md on first open and copies it whole.
• site-nav.js loads agent-modal.js and agent-modal.css on hover, focus or click, so no page loads them up front.
• From 1080 to 1239 px the bar links pack tighter, so the bar stays 1006 px.
• README and agent.md say where to find it. llms-full.txt is rebuilt.