Agent integration: new docs page + sidebar entry across all docs

Ports the in-repo KEYSAT_AGENT_GUIDE.md into the docs site as a
first-class page rather than linking out to a raw markdown file on
GitHub. The page covers authentication, scoped API keys, OpenAPI
discovery, error envelope conventions, common workflows (issue /
revoke / find / cancel / change-tier / free-machine), webhook
event types + signature verification, robust-agent patterns, a
"comp-license-via-email" recipe, and the operator-only
operations that aren't exposed to any scoped key.

Sidebar gains an "Agent integration" entry under Get started on
every page (index, install, integrate, wire-format, operate, agent
itself). Docs index "These docs cover" + "Where to next" grids
each gain a third card pointing at the agent guide so it's
discoverable from the introduction page even for visitors who
don't scan the sidebar.

install.html

@@ -21,6 +21,7 @@
       <a href="index.html">Introduction</a>
       <a href="install.html" class="active">Install &amp; setup</a>
       <a href="integrate.html">Integrate the SDK</a>
+      <a href="agent.html">Agent integration</a>
     </div>
     <div class="group">
       <div class="glabel">Concepts</div>