feat: add frontend customization UI and prompt picker

Add Copilot Customization section to ProjectSettingsPanel with instruction file
previews, agent profile enable/disable toggles, and prompt file listing. Add
InlinePromptPill to chat input for selecting and sending prompt files with
variable substitution. Wire rescan and agent profile IPC in App.tsx. Update
website with Copilot Customization feature card.

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

website/src/pages/index.astro

@@ -262,6 +262,19 @@ import Base from '../layouts/Base.astro';
             Every turn is recorded in a structured run timeline — see tool calls, agent delegation, hook execution, and context usage at a glance.
           </p>
         </div>
+
+          <!-- Feature 14: Copilot Customization Files -->
+          <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-violet-500/10 text-violet-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="M11 5H6a2 2 0 00-2 2v11a2 2 0 002 2h11a2 2 0 002-2v-5m-1.414-9.414a2 2 0 112.828 2.828L11.828 15H9v-2.828l8.586-8.586z" />
+              </svg>
+            </div>
+            <h3 class="text-base font-semibold">Copilot Customization</h3>
+            <p class="mt-2 text-sm leading-relaxed text-zinc-400">
+              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>
       </div>
     </div>
   </section>