agent-team v0.5.1 — 订阅模型单 agent 兜底（solo fallback）

When the Lead's confirmed model is an official coding-plan model (account:*/builtin:*), teammates cannot be spawned (CLI subprocesses get no subscription credentials). 0.5.1 tells the Lead once per session — it then does all the work itself instead of burning turns on doomed spawns. The note is non-permanent: switch the Lead to a third-party model mid-session and the team works normally again. Spawn errors and the spawn tool description now offer the same fallback path.

中文要点（issue #5）

• Lead 用官方套餐模型时，会话开始（或首轮提问）一次性告知「队友不可用，自己干」
• 中途切第三方模型后 spawn 成功即恢复正常用队；检测乐观，绝不误报
• 882 测试全绿（+13）、文本审计 25 项、真实订阅会话库端到端实证

zip sha256: 53c768a7da91c89349d7b1ea8d5ebb389c6bcec604c7c354a167ab8ffe021509