feat: llms.txt for AI agents, plus five docs-site fixes (0.1.17-alpha)

[Shewart]

llms.txt for AI agents

o.EnableLlmsTxt(l => ...) publishes a ShellDocs site for coding agents (ported from the ShellUI docs site and made generic):

• /llms.txt lists every page with its description, grouped by sidebar section. /llms-full.txt has every page as Markdown. Each page is also served as Markdown at its URL + .md. Each version root gets the same files.
• AGENTS.md, CLAUDE.md, copilot-instructions.md and a Cursor rule are served as downloads under agents/. Each is a ## {SiteName} section, so it can be appended to an existing file.
• Options: Summary, RulesFile (shown at the top), InstallCommandPattern (a regex; the first match in a page's code blocks is listed after it), AgentFiles, PageActions.
• The files are served as text/plain, and the agent files as downloads. Middleware answers these paths before routing, so they don't clash with the docs page route. Links use SiteUrl, or the request's address.
• ShellDocs tags become Markdown: demos become their source, callouts become blockquotes (fences inside are kept), tabs get a bold label per tab, and cards become a list. Steps, file trees and type tables are converted too. Other registered components become their text. Links to docs pages become absolute .md links. Tags inside code fences are left alone.
• A ⋯ menu by the page title: Copy page, View as Markdown, Open in Claude, Open in ChatGPT. shelldocs.js drives it, so it works in static exports.
• Links to .txt and .md files load the file instead of the docs' "Page not found".
• shelldocs build writes all the files into the export. Links use --site-url, then SiteUrl, otherwise they're root-relative under --base-href.
• shelldocs init turns it on. The example site uses it and has an llms.txt page. It's documented in the README and ARCHITECTURE.

Fixes

• The missing-tab note keeps its spaces.
• razor:preview fences that hold only HTML render live.
• 404.html shows the visitor's path instead of the probe URL.
• LogoSvg logos are sized to LogoHeight.
• HomeLayout includes <SearchDialog />.

Checked

• Every commit builds in Release with 0 warnings and passes its tests on its own. 367 tests in total, 12 new, including one that converts every Docs* tag and one that fails if a built-in component has no Markdown form.
• Example site: the endpoints, content types, the download header, an unknown .md returning 404, the converted pages, the menu and the chat links. A .md link loads the file while the Blazor circuit is running.
• Static export served by python -m http.server: 44 llms files written with root-relative links. The menu, Copy page and View as Markdown work, and the /llms.txt link loads the file.
• Copy page was checked with a stubbed clipboard, because the test browser wasn't focused.