feat(agent): foldable Vue terminal with Ctrl+O toggle

Replaces the xterm.js scrollback with a Vue-native terminal that can
fold tool-call output. Tool calls collapse to a one-line summary by
default — "$ <cmd>   N lines, exit M" with green/red exit-code
colour — and Ctrl+O (or ⌘+O) toggles fold/unfold across every block at
once. Click the block header to toggle a single tool call.

Why this trade-off:
- xterm.js is a character grid; folding requires a parallel logical
  model and a full re-render on every toggle. Vue components track
  semantic blocks natively, so fold state is just reactive data.
- Side benefits: per-block text selection/copy, native theming via
  Tailwind tokens, no ANSI cursor math, no @xterm/* deps in the agent
  bundle, easier to extend (per-tool icons, inline images, links).

Implementation notes:
- store.AgentMessage gains an optional 'tool' meta field carrying
  script/stdout/stderr/exitCode so the renderer doesn't have to parse
  the synthesized text summary back into structure.
- Global keydown listener with capture:true preempts the browser's
  default Open-File dialog before Ctrl+O reaches it.
- Drops src/agent/ui/XtermPanel.vue, useXtermReadline.ts, and
  shell/useCompletion.ts. The xterm package is still used elsewhere
  in the codebase (logs/CommandTerminal), so no dep removal.

Author: snomiao

src/agent/composables/useAgentSession.ts

@@ -187,7 +187,16 @@ export function useAgentSession() {
           if (ac.signal.aborted) return
           toolCalls.push(inv)
           const summary = `$ ${inv.script}\n${inv.stdout}${inv.stderr ? `\n[stderr] ${inv.stderr}` : ''}`
-          store.addMessage({ role: 'system', text: summary })
+          store.addMessage({
+            role: 'system',
+            text: summary,
+            tool: {
+              script: inv.script,
+              stdout: inv.stdout,
+              stderr: inv.stderr,
+              exitCode: inv.exitCode
+            }
+          })
         }
       )
       // Fallback: model ran tools but didn't speak — surface a minimal

src/agent/shell/useCompletion.ts

@@ -16,12 +16,25 @@ export interface IngestedAsset {
   previewUrl?: string
 }
 
+interface ToolMessageMeta {
+  script: string
+  stdout: string
+  stderr?: string
+  exitCode: number
+}
+
 interface AgentMessage {
   id: string
   role: AgentMessageRole
   text: string
   assets?: IngestedAsset[]
   createdAt: number
+  /**
+   * Present on system messages that record a tool invocation. Lets the
+   * renderer fold/unfold individual tool calls by structure instead of
+   * re-parsing the synthesized text summary.
+   */
+  tool?: ToolMessageMeta
 }
 
 interface FabPosition {

src/agent/stores/agentStore.ts

@@ -1,7 +1,7 @@
 <template>
   <Teleport to="body">
     <AgentFab />
-    <XtermPanel />
+    <FoldablePanel />
   </Teleport>
 </template>
 
@@ -14,7 +14,7 @@ import { useCommandStore } from '@/stores/commandStore'
 
 import { useAgentStore } from '../stores/agentStore'
 import AgentFab from './AgentFab.vue'
-import XtermPanel from './XtermPanel.vue'
+import FoldablePanel from './FoldablePanel.vue'
 
 onMounted(() => {
   const commandStore = useCommandStore()