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

Root-relative links (/docs/introduction) ignore <base href>, so the
sidebar, breadcrumb, header, prev/next, content links and favicons of an
export built with --base-href /my-repo/ pointed outside the site.
shelldocs build now prefixes root-relative href, src and data-src values
in every exported page.

Fixes #40

fix(cli): put --base-href in front of root-relative links

Fixes #41

SearchState, SidebarCollapseState and MobileNavState have had no users since search, the sidebar collapse and the mobile drawer moved to shelldocs.js.

Closes #46

Fixes #42

Serves the output of shelldocs build the way static hosts do: folders serve their index.html, unknown URLs get 404.html with a 404, and a --base-href export is served under its path (read from the export by default). Replaces the unimplemented 'preview <name>' stub.

Fixes #43

Graphite (the existing palette, still the default), Harbor, Lagoon, Forest, Savanna, Ember and Jacaranda, each in light and dark with text at 4.5:1 or better. The layouts render data-shelldocs-theme and tokens.css sets every colour token from it, at the specificity of :root so site overrides still win. The active sidebar item, its marker and link underlines follow the brand tokens. shelldocs init --theme takes the preset names. Shadcn, Fuma and Nextra stay as obsolete aliases of Graphite.

Fixes #44

All six packages shipped the GitHub README, whose centred <picture> header and badges are HTML that nuget.org prints as text. Each project now packs its own README.md, laid out like ShellUI's. pack-dry-run fails on HTML in a package README, and the PowerShell version runs on Windows PowerShell 5.1 again (ASCII dashes, ZipFile instead of Expand-Archive).

Fixes #45

feat: colour presets, shelldocs preview, NuGet READMEs and the issue backlog (0.1.18-alpha)

Under <Routes @rendermode="InteractiveServer" /> the circuit replaced the
prerendered TOC and headings, so the scroll-spy watched detached elements
and the highlight stopped moving. TOC links were bare #id, which resolves
against <base href> to the home page.

- TOC links carry the page's base-relative path (docs/setup#install)
- the scroll-spy re-attaches when a render replaces the list or headings,
  or changes data-toc-ids on navigation
- same-page links (#id in content too) scroll in place and keep the path
- at the bottom of the page the last heading on screen is active

The unlayered *, *::before, *::after rule in shelldocs-theme.css beat Tailwind's border-colour utilities (@layer utilities) in previews and home pages, so border-destructive rendered as --border. It's in @layer base now.

Top-level pages on both sides of a folder gave two "## Pages" groups; groups are now listed once, in first-seen order. The page menu's chat prompts used the visitor's origin, so an export previewed on localhost named a localhost URL; with SiteUrl set they name the published page.

fix: table of contents on interactive sites, border layer, llms.txt groups

chore: release 0.1.19-alpha