feat: zero-friction side panel — auto-open on install, pill is clickable

Three changes to eliminate manual side panel setup:
- Auto-open side panel on extension install/update (onInstalled listener)
- gstack pill (bottom-right) is now clickable — opens the side panel
- Pill has pointer-events: auto so clicks always register (was: none)

User no longer needs to find the puzzle piece icon, pin the extension,
or know the side panel exists. It opens automatically on first launch
and can be re-opened by clicking the floating gstack pill.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

extension/content.js

@@ -19,6 +19,11 @@ function showStatusPill(connected, refs) {
   if (!statusPill) {
     statusPill = document.createElement('div');
     statusPill.id = 'gstack-status-pill';
+    statusPill.style.cursor = 'pointer';
+    statusPill.addEventListener('click', () => {
+      // Ask background to open the side panel
+      chrome.runtime.sendMessage({ type: 'openSidePanel' });
+    });
     document.body.appendChild(statusPill);
   }