fix(pi-extension): guard status bar render when ui has no theme (#279)

syncStatus guarded setStatus but used theme.fg unguarded, so a Pi host
exposing setStatus without a theme threw TypeError on session_start (and
agent_start/agent_end/setMode). Require both before rendering.

Add tests for the render path (previously untested) and the theme-absent
degradation. Follow-up to #275 / #84.

pi-extension/index.js

@@ -63,7 +63,7 @@ export default function ponytailExtension(pi) {
   function syncStatus(ctx) {
     if (ctx) lastCtx = ctx;
     const c = ctx || lastCtx;
-    if (!c?.ui?.setStatus) return;
+    if (!c?.ui?.setStatus || !c.ui.theme?.fg) return;
     const theme = c.ui.theme;
     if (currentMode === "off") {
       c.ui.setStatus("ponytail", "");