feat: add integrated terminal frontend

- Add TerminalPanel component with xterm.js, FitAddon, drag-to-resize,
  header bar (status dot, shell label, cwd, restart/minimize/close)
- Modify AppShell to accept terminal panel in vertical flex layout
- Add terminal state management in App.tsx (open/height/running state,
  Ctrl+backtick toggle, height persistence via IPC)
- Add InlineTerminalPill to composer pill row (both single/multi-agent)
- Install @xterm/xterm and @xterm/addon-fit as devDependencies
- Update ARCHITECTURE.md with frontend terminal component details
- Add Integrated Terminal feature card to marketing website

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

website/src/pages/index.astro

@@ -275,6 +275,19 @@ import Base from '../layouts/Base.astro';
               Automatically discovers <code class="text-zinc-300">.github/copilot-instructions.md</code>, <code class="text-zinc-300">AGENTS.md</code>, custom agent profiles, and prompt files from your repository. Zero configuration needed.
             </p>
           </div>
+
+          <!-- Feature 15: Integrated Terminal -->
+          <div class="group rounded-2xl border border-zinc-800 bg-zinc-900/40 p-6 transition hover:border-zinc-700 hover:bg-zinc-900/60">
+            <div class="mb-4 flex size-10 items-center justify-center rounded-xl bg-emerald-500/10 text-emerald-400">
+              <svg class="size-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M8 9l3 3-3 3m5 0h3M5 20h14a2 2 0 002-2V6a2 2 0 00-2-2H5a2 2 0 00-2 2v12a2 2 0 002 2z" />
+              </svg>
+            </div>
+            <h3 class="text-base font-semibold">Integrated Terminal</h3>
+            <p class="mt-2 text-sm leading-relaxed text-zinc-400">
+              A full PTY-backed terminal panel right inside the workspace. Toggle with <kbd class="text-zinc-300">Ctrl+`</kbd>, resize by dragging, and run commands alongside your agent sessions.
+            </p>
+          </div>
       </div>
     </div>
   </section>