feat(slash-commands): expose Claude Code CLI built-ins in command menu

Adds /simplify, /loop, and /batch as CLI-sourced slash commands so they
appear in the autocomplete menu with a 'CLI' badge and forward correctly
to Claude Code instead of triggering 'Unknown command' errors.

Introduces source?: 'cli' field to the SlashCommand interface to
distinguish app-level commands from CLI pass-through entries. Resolves
#208.

src/lib/components/SlashCommandMenu.svelte

@@ -23,6 +23,9 @@
       >
         <span class="command-name">/{command.name}</span>
         <span class="command-description">{command.description}</span>
+        {#if command.source === "cli"}
+          <span class="cli-badge">CLI</span>
+        {/if}
       </button>
     {/each}
   </div>
@@ -82,5 +85,19 @@
   .command-description {
     color: var(--text-secondary);
     font-size: 13px;
+    flex: 1;
+  }
+
+  .cli-badge {
+    font-size: 10px;
+    font-weight: 600;
+    padding: 1px 5px;
+    border-radius: 4px;
+    background: color-mix(in srgb, var(--accent-primary) 15%, transparent);
+    color: var(--accent-primary);
+    border: 1px solid color-mix(in srgb, var(--accent-primary) 30%, transparent);
+    letter-spacing: 0.5px;
+    text-transform: uppercase;
+    flex-shrink: 0;
   }
 </style>