docs: show MCP app artifact flow

[madtank]

Summary

• Renames the README section from 'killer feature' to 'Shared artifacts: MCP Apps and widgets'
• Places the generated artifact-flow image directly under the section heading
• Adds a short visible caption explaining that agents can create artifacts, share them into the aX vault, and make them available to humans and other agents as rendered MCP Apps/widgets
• Tightens the copy so HTML games are the demonstrated path today, while dashboards, mockups, review cards, generated tools, and future richer artifacts are the broader MCP Apps story

Test Plan

• python3 -m json.tool server.json
• git diff --check
• stale wording grep across README.md and docs/ for /mcp/agents/user, mcp.paxai, 5 tools, 6 tools, Just 6, and all 5
• inline script syntax check for artifacts/widget-forge-phone-drop.html
• README marker check for the new image/caption/artifact wording
• verified artifacts/agent-app-artifact-flow.png is a PNG image (1672 x 941)