feat: per-agent tools toggle and better python scratchpad

Add ToolSelector for enabling sandboxed python per agent, drop unsafe
filesystem/bash tools, bundle fetchUrl with web search, and return
Monty's last expression value so agents need not print.

app/components/ToolSelector.vue

@@ -0,0 +1,45 @@
+<script setup lang="ts">
+const props = defineProps<{
+    python: boolean;
+}>();
+
+const emit = defineEmits<{
+    'update:python': [value: boolean];
+}>();
+
+const anyEnabled = computed(() => props.python);
+</script>
+
+<template>
+    <Dropdown dropdownClass="text-sm" placement="top">
+        <template #default="{ toggle, setRef }">
+            <button :ref="setRef" @click="toggle"
+                class="flex items-center justify-center h-8.5 w-8.5 @hover:bg-[var(--color-hover)] rounded-lg transition-colors duration-200 ease-[cubic-bezier(0.5,_1,_0.89,_1)]">
+                <span class="pointer-events-none i-mynaui-archive text-5 transition-colors duration-200"
+                    :class="anyEnabled ? 'text-[var(--color-accent)]' : 'text-[var(--text-secondary)]'"></span>
+            </button>
+        </template>
+
+        <template #dropdown>
+            <div class="flex flex-col gap-1 p-1 min-w-48">
+                <label
+                    class="flex items-start gap-3 rounded-xl px-3 py-2.5 text-left cursor-pointer transition-colors duration-150 @hover:bg-[var(--color-hover)]"
+                    :class="python ? 'bg-[var(--color-active)]' : ''">
+                    <input type="checkbox" :checked="python"
+                        @change="emit('update:python', ($event.target as HTMLInputElement).checked)"
+                        class="mt-0.5 w-4 h-4 rounded border-[var(--color-border)] bg-transparent text-[var(--color-accent)] focus:ring-[var(--color-accent)] focus:ring-offset-0 cursor-pointer shrink-0" />
+                    <div class="flex flex-col gap-0.5">
+                        <span class="text-sm font-medium"
+                            :class="python ? 'text-[var(--text-primary)]' : 'text-[var(--text-secondary)]'">
+                            Python
+                        </span>
+                        <span class="text-xs leading-snug"
+                            :class="python ? 'text-[var(--text-secondary)]' : 'text-[var(--text-dim)]'">
+                            Run code in a sandbox
+                        </span>
+                    </div>
+                </label>
+            </div>
+        </template>
+    </Dropdown>
+</template>