docs: document the OpenAPI spec + Swagger UI, add H0M3 API tile

Closes #2965.

hive-c0re auto-generates an OpenAPI 3 spec via utoipa
(hive-c0re/src/dashboard/mod.rs's ApiDoc), served raw at
/api/openapi.json and browsable as a Swagger UI at /api/docs, but
docs/ never mentioned either — genuinely zero hits grepping the whole
docs/ tree. Documented both in docs/web-ui/dashboard.md's Dashboard
endpoints + H0M3 page sections.

Also added the H0M3 hub's API tile mara suggested ("maybe also add
home page app that opens swagger ui") -- a plain static link to
/api/docs, no gating needed since the endpoint always exists
(unlike Forge/Matrix, which are conditionally enabled).

frontend/packages/dashboard/src/index.html

@@ -100,6 +100,17 @@
         <span class="home-tile-desc">provision per-agent matrix + github accounts</span>
       </a>
 
+      <!-- API tile: no gating, unlike Forge/Matrix below — the OpenAPI
+           spec + Swagger UI are always served by hive-c0re itself
+           (docs/web-ui/dashboard.md::Dashboard endpoints). -->
+      <a class="home-tile" href="/api/docs">
+        <span class="home-tile-head">
+          <span class="home-tile-icon" aria-hidden="true">🧬</span>
+          <span class="home-tile-label">API</span>
+        </span>
+        <span class="home-tile-desc">interactive OpenAPI spec (Swagger UI)</span>
+      </a>
+
       <!-- Forge tile: hidden until home.js confirms the hive-forge
            container is up (state.forge_present); home.js also fills the
            href from state.forge_public_url (or the :3000 fallback), so