docs: clarify botmux ask fallback send contract

[deepcoldy]

Summary

• Clarify that botmux ask buttons only returns the selected key to stdout and does not automatically send a visible Lark reply.
• Update the fallback botmux-ask canonical example to capture the selection and call botmux send --mention-back.
• Add a built-in skill regression assertion for the stdout/send contract.

Tests

• pnpm vitest run test/builtin-skills.test.ts test/ensure-ask-skill.test.ts