feat(cli): add /history slash command to TUI

Adds /history (alias /hist) as a local TUI command that fetches the
current conversation's messages via GET /api/conversations/:id/messages
and displays message counts, user/assistant breakdown, estimated token
usage (~4 chars/token heuristic), and context window percentage.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/cli/src/tui/gateway-api.ts

@@ -361,6 +361,31 @@ export async function deleteMission(
   }
 }
 
+// ── Conversation Message types ──
+
+export interface ConversationMessage {
+  id: string;
+  role: 'user' | 'assistant' | 'system' | 'tool';
+  content: string;
+  createdAt: string;
+}
+
+// ── Conversation Message endpoints ──
+
+export async function fetchConversationMessages(
+  gatewayUrl: string,
+  sessionCookie: string,
+  conversationId: string,
+): Promise<ConversationMessage[]> {
+  const res = await fetch(
+    `${gatewayUrl}/api/conversations/${encodeURIComponent(conversationId)}/messages`,
+    {
+      headers: headers(sessionCookie, gatewayUrl),
+    },
+  );
+  return handleResponse<ConversationMessage[]>(res, 'Failed to fetch conversation messages');
+}
+
 // ── Mission Task endpoints ──
 
 export async function fetchMissionTasks(