Auto-render Slack ask_user questions as buttons when options are simple

[LIU9293]

Summary

• Moves the "send buttons vs plain text" decision out of the agent system prompt and into the Slack IM adapter — Ode now auto-renders interactive Slack buttons whenever a question's options qualify as "simple" (2-5 labels, each ≤ 15 chars, no newlines), and falls back to the existing Options: a / b / c plain text otherwise.
• Both entry points share one renderer: the SDK-emitted question.asked event (Path B, previously always plain text) and the LLM-invoked /api/action ask_user (Path A, previously 2-5 options only, hard-errored otherwise). The action API no longer rejects >5 options; they just render as text.
• Drops the Slack-specific ask_user/buttons guidance line from buildSystemPrompt — the LLM doesn't need to know about it anymore.

Changes

• packages/core/runtime/helpers.ts — new hasSimpleOptions() heuristic.
• packages/core/types.ts — IMAdapter gains an optional sendQuestion(channelId, threadId, question, options, prefix?) hook.
• packages/ims/slack/api.ts — extract postSlackQuestion() (blocks when simple, plain text otherwise); simplify ask_user case to reuse it.
• packages/ims/slack/client.ts — Slack adapter implements sendQuestion via a lazy import of ./api to avoid circular deps.
• packages/core/kernel/request-run.ts and pending-question.ts — prefer im.sendQuestion when available, fall back to existing sendMessage + formatSingleQuestionPrompt for other platforms.
• packages/agents/shared.ts — remove the "use ask_user / buttons are enough" Slack system-prompt line.
• Unit tests for hasSimpleOptions edge cases (len/newline/bounds) and for the multi-question follow-up using sendQuestion.

Testing

• bun test packages/core packages/agents packages/ims → 211 pass, 0 fail.
• Manual: posted ask_user with 3 short options to a live thread (rendered as buttons) and with a long-label option (fell back to plain text).

Discord and Lark adapters don't implement sendQuestion, so they keep the existing plain-text behavior unchanged.