agent: add ?hide= URL param to suppress header/composer chrome

For embedding companions (e.g. trollshell WebKitGTK) that supply their
own header/input chrome around the per-agent page and only want the
raw terminal feed. Comma-separated so both axes can hide in one param:
?hide=header,input. Zeroes --agent-header-h/--agent-composer-h via body
classes rather than conditionally computing padding, so every dependent
calc() (overlay offset, terminal scroll padding) collapses in one place.

frontend/packages/agent/src/agent.css

@@ -30,6 +30,19 @@
   --agent-frost-blur: blur(12px) saturate(140%);
 }
 
+/* `?hide=header,input` (per-agent-page embed mode): `Root.tsx` toggles
+   these body classes when it omits `<Header>`/the composer footer.
+   Zeroing the custom properties here — rather than overriding each
+   dependent rule — collapses every calc() that reads them (overlay
+   offset, terminal scroll padding) in one place. */
+body.agent-hide-header {
+  --agent-header-h: 0px;
+  --agent-header-real-h: 0px;
+}
+body.agent-hide-input {
+  --agent-composer-h: 0px;
+}
+
 html,
 body {
   height: 100%;