feat: show remaining context window for Claude Code, Codex, and OpenCode

Track and display context window usage across all three agent providers,
letting users see how much context remains before hitting limits.

packages/server/src/shared/messages.ts

@@ -146,6 +146,8 @@ const AgentUsageSchema: z.ZodType<AgentUsage> = z.object({
   cachedInputTokens: z.number().optional(),
   outputTokens: z.number().optional(),
   totalCostUsd: z.number().optional(),
+  contextWindowMaxTokens: z.number().optional(),
+  contextWindowUsedTokens: z.number().optional(),
 });
 
 const McpStdioServerConfigSchema = z.object({