feat: prepare Audrey 0.21 release

[Evilander]

Summary

• adds audrey doctor first-contact diagnostics with JSON automation output
• adds safe install --host <host> --dry-run setup previews for Codex, Claude Code, and generic MCP hosts
• rewrites the README as the product front door and adds a committed Audrey SVG logo
• bumps npm package/runtime version to 0.21.0 and includes release docs/assets in the package

Local Validation

• npm run build passed
• npm run typecheck passed
• npm run bench:memory:check passed: Audrey 100.0%, ahead of Vector Only by 58.3 points
• npm audit --audit-level=moderate --cache .\.tmp-npm-cache passed with 0 vulnerabilities
• node dist\mcp-server\index.js doctor passed with Verdict: ready
• node dist\mcp-server\index.js install --host codex --dry-run passed
• node dist\mcp-server\index.js install --host claude-code --dry-run passed
• node dist\mcp-server\index.js demo passed
• npm pack --dry-run --cache .\.tmp-npm-cache passed and includes docs/assets/audrey-logo.svg

Note

Local npm test is blocked before tests start by the known Windows host spawn EPERM from Vite/esbuild. GitHub Actions is the authoritative Vitest gate for this PR.