feat: expose prompt caching TTL setting in UI (#273)

Adds a dropdown to select ENABLE_PROMPT_CACHING_1H or FORCE_PROMPT_CACHING_5M env vars when starting Claude Code sessions. Requires Claude Code v2.1.108+.

src/lib/components/ConfigSidebar.svelte

@@ -79,6 +79,7 @@
     task_loop_commit_prefix: "feat",
     task_loop_include_summary: false,
     effort_level: null,
+    prompt_caching_ttl: null,
   });
 
   let showCustomThemeEditor = $state(false);
@@ -740,6 +741,26 @@
         </p>
       </div>
 
+      <!-- Prompt Caching TTL -->
+      <div class="mb-4">
+        <label class="block text-sm text-[var(--text-primary)] mb-1" for="prompt-caching-ttl"
+          >Prompt caching TTL</label
+        >
+        <select
+          id="prompt-caching-ttl"
+          bind:value={config.prompt_caching_ttl}
+          class="w-full px-3 py-2 rounded border border-[var(--border-color)] bg-[var(--bg-primary)] text-[var(--text-primary)] text-sm focus:outline-none focus:ring-1 focus:ring-[var(--accent-primary)]"
+        >
+          <option value={null}>Default (CLI decides)</option>
+          <option value="1h">1-hour TTL (reduces cost for long sessions)</option>
+          <option value="5m">Force 5-minute TTL</option>
+        </select>
+        <p class="text-xs text-[var(--text-tertiary)] mt-1">
+          Sets <code class="font-mono">ENABLE_PROMPT_CACHING_1H</code> or
+          <code class="font-mono">FORCE_PROMPT_CACHING_5M</code>. Requires Claude Code v2.1.108+.
+        </p>
+      </div>
+
       <!-- Bare Mode -->
       <div class="mb-4">
         <label class="flex items-center gap-3 cursor-pointer">