nix/docs: extract CSS into sibling file (#625)

Per mara on #622 comment 7442:
> follow up moving scripts and css and stuff out of the nix file.
> can live in the same dir.

Layout:

    nix/docs/
      default.nix   ← what was nix/docs.nix
      style.css     ← extracted from inline `styleCSS = '' ... ''`

`builtins.readFile ./style.css` loads the stylesheet at evaluation
time, so the rendered HTML stays byte-identical (CSS inlined into
each page's `<style>` block — verified). Future client-side scripts
can land at `nix/docs/script.js` with the same `builtins.readFile`
pattern.

Bonus: the docs.nix stub NixOS eval was still force-disabling
`hyperhive.{forge,matrix}.enable` on the pre-#615 namespace; updated
to `services.hyperhive.{forge,matrix}.enable` so `nix flake check`
passes against current main. (Same fix lives on PRs #619 + #620;
whichever lands first wins, the others rebase to a no-op.)

`flake.nix` references updated: `./nix/docs.nix` → `./nix/docs`.

Verified:
- `nix flake check --no-build` passes clean
- `nix build .#docs` produces 5-file bundle identical to pre-PR shape
- inline CSS still appears 3× per HTML page (one per index/host/agent)

nix/docs/style.css

@@ -0,0 +1,76 @@
+:root {
+  color-scheme: light dark;
+  --fg: #1a1a1a;
+  --bg: #fafafa;
+  --muted: #6b6b6b;
+  --accent: #5e548e;
+  --code-bg: rgba(94, 84, 142, 0.08);
+  --rule: rgba(94, 84, 142, 0.2);
+}
+@media (prefers-color-scheme: dark) {
+  :root {
+    --fg: #e6e6e6;
+    --bg: #0d0d0d;
+    --muted: #9b9b9b;
+    --accent: #c4a7e7;
+    --code-bg: rgba(196, 167, 231, 0.08);
+    --rule: rgba(196, 167, 231, 0.2);
+  }
+}
+* { box-sizing: border-box; }
+body {
+  font-family: system-ui, -apple-system, "Segoe UI", Roboto, sans-serif;
+  max-width: 56rem;
+  margin: 0 auto;
+  padding: 2rem 1.25rem 4rem;
+  color: var(--fg);
+  background: var(--bg);
+  line-height: 1.55;
+}
+nav {
+  margin-bottom: 2rem;
+  padding-bottom: 1rem;
+  border-bottom: 1px solid var(--rule);
+  font-size: 0.9rem;
+  color: var(--muted);
+}
+nav a { color: var(--accent); text-decoration: none; }
+nav a:hover { text-decoration: underline; }
+nav a + a { margin-left: 0.5rem; }
+nav a + a::before { content: "· "; color: var(--muted); margin-right: 0.25rem; }
+h1, h2, h3 { color: var(--accent); line-height: 1.25; }
+h1 { font-size: 1.75rem; margin-top: 0; }
+h2 {
+  font-size: 1.15rem;
+  margin-top: 2.5rem;
+  padding-top: 0.5rem;
+  border-top: 1px solid var(--rule);
+  font-family: ui-monospace, "SFMono-Regular", Menlo, monospace;
+}
+h3 { font-size: 1rem; }
+p { margin: 0.75rem 0; }
+code {
+  background: var(--code-bg);
+  padding: 0.1em 0.35em;
+  border-radius: 0.25em;
+  font-family: ui-monospace, "SFMono-Regular", Menlo, monospace;
+  font-size: 0.9em;
+}
+pre {
+  background: var(--code-bg);
+  padding: 0.85rem 1rem;
+  border-left: 2px solid var(--accent);
+  border-radius: 0 0.25rem 0.25rem 0;
+  overflow-x: auto;
+  line-height: 1.4;
+}
+pre code { background: transparent; padding: 0; }
+a { color: var(--accent); }
+em { color: var(--muted); font-style: normal; font-size: 0.9em; }
+footer {
+  margin-top: 4rem;
+  padding-top: 1rem;
+  border-top: 1px solid var(--rule);
+  font-size: 0.85rem;
+  color: var(--muted);
+}