feat(extension): Terminal as default sidebar tab

Adds a primary tab bar (Terminal | Chat) above the existing tab-content
panes. Terminal is the default-active tab; clicking Chat returns to the
existing claude -p one-shot flow which is preserved verbatim.

manifest.json: adds ws://127.0.0.1:*/ to host_permissions so MV3 doesn't
block the WebSocket upgrade.

sidepanel.html: new primary-tabs nav, new #tab-terminal pane with a
"Press any key to start Claude Code" bootstrap card, claude-not-found
install card, xterm mount point, and "session ended" restart UI. Loads
xterm.js + xterm-addon-fit + sidepanel-terminal.js. tab-chat is no
longer the .active default.

sidepanel.js: new activePrimaryPaneId() helper that reads which primary
tab is selected. Debug-close paths now route back to whichever primary
pane is active (was hardcoded to tab-chat). Primary-tab click handler
toggles .active classes and aria-selected. window.gstackServerPort and
window.gstackAuthToken exposed so sidepanel-terminal.js can build the
/pty-session POST and the WS URL.

sidepanel-terminal.js (new): xterm.js lifecycle. Lazy-spawn — first
keystroke fires POST /pty-session, then opens
ws://127.0.0.1:<terminalPort>/ws. Origin + cookie are set automatically
by the browser. Resize observer sends {type:"resize"} text frames.
ResizeObserver, tab-switch hooks, restart button, install-card retry.
On WS close shows "Session ended, click to restart" — no auto-reconnect
(codex outside-voice flagged that as session-burning).

sidepanel.css: primary-tabs bar + Terminal pane styling (full-height
xterm container, install card, ended state).

extension/manifest.json

@@ -4,7 +4,7 @@
   "version": "0.1.0",
   "description": "Live activity feed and @ref overlays for gstack browse",
   "permissions": ["sidePanel", "storage", "activeTab", "scripting"],
-  "host_permissions": ["http://127.0.0.1:*/"],
+  "host_permissions": ["http://127.0.0.1:*/", "ws://127.0.0.1:*/"],
   "action": {
     "default_icon": {
       "16": "icons/icon-16.png",