docs(options): emit markdown only, render HTML in website

Per #1407 review: the options reference should be markdown here; the
HTML + CSS belongs in the website repo where the theme lives and the
stylesheet can be shared with /docs.

- nix/docs/default.nix: drop the cmark-gfm HTML rendering, the inline
  <style>, and the HTML index. Emit index.md + host.md + agent.md only
  (CommonMark from nixosOptionsDoc).
- Remove nix/docs/style.css.
- Update docs/gotchas.md + CLAUDE.md to describe the md-only output and
  point at the website for HTML rendering.

The website (hyperhive/website#26) renders these .md to themed HTML for
/options/, sharing one docs.css with the prose /docs/ tree.

CLAUDE.md

@@ -348,10 +348,10 @@ nix/
                                  — weston + VNC backend systemd unit; writes
                                  /etc/hyperhive/gui.json (vnc_port + auth) for
                                  the harness WebSocket relay (/screen/ws)
-  docs/                          HTML+MD options reference (host + agent); built
-    default.nix                  via `nix build .#docs` → static site
-    style.css                    (`index.html`, `host.html`, `agent.html`,
-                                 `host.md`, `agent.md`); rendered by cmark-gfm
+  docs/                          markdown options reference (host + agent);
+    default.nix                  `nix build .#docs` → `index.md` + `host.md`
+                                 + `agent.md` (CommonMark from nixosOptionsDoc).
+                                 HTML+CSS rendered downstream by the website repo
   forge-theme/theme-catppuccin-vibec0re.css  Catppuccin Mocha forge theme
 
 docs/