fix(ce-brainstorm): enforce Interaction Rules in universal flow (#669)

plugins/compound-engineering/skills/ce-brainstorm/SKILL.md

@@ -27,10 +27,13 @@ This skill does not implement code. It explores, clarifies, and documents decisi
 
 ## Interaction Rules
 
-1. **Ask one question at a time** - Do not batch several unrelated questions into one message.
+These rules apply to every brainstorm, including the universal (non-software) flow routed to `references/universal-brainstorming.md`.
+
+1. **Ask one question at a time** - One question per turn, even when sub-questions feel related. Stacking several questions in a single message produces diluted answers; pick the single most useful one and ask it.
 2. **Prefer single-select multiple choice** - Use single-select when choosing one direction, one priority, or one next step.
 3. **Use multi-select rarely and intentionally** - Use it only for compatible sets such as goals, constraints, non-goals, or success criteria that can all coexist. If prioritization matters, follow up by asking which selected item is primary.
-4. **Use the platform's blocking question tool** - `AskUserQuestion` in Claude Code (call `ToolSearch` with `select:AskUserQuestion` first if its schema isn't loaded), `request_user_input` in Codex, `ask_user` in Gemini, `ask_user` in Pi (requires the `pi-ask-user` extension). Fall back to numbered options in chat only when no blocking tool exists in the harness or the call errors (e.g., Codex edit modes) — not because a schema load is required. Never silently skip the question.
+4. **Default to the platform's blocking question tool** - Use `AskUserQuestion` in Claude Code (call `ToolSearch` with `select:AskUserQuestion` first if its schema isn't loaded), `request_user_input` in Codex, `ask_user` in Gemini, `ask_user` in Pi (requires the `pi-ask-user` extension). These tools include a free-text fallback (e.g., "Other" in Claude Code), so options scaffold the answer without confining it — well-chosen options surface dimensions the user may not have separated, and pick-plus-optional-note is lower activation energy than composing prose from scratch. This default holds for opening and elicitation questions too, not only narrowing. Fall back to numbered options in chat only when no blocking tool exists in the harness or the call errors (e.g., Codex edit modes) — not because a schema load is required. Never silently skip the question.
+5. **Use prose only when the question is genuinely open** - Drop the blocking tool only when (a) the answer is inherently narrative ("walk me through how you got here"), (b) the question is diagnostic or introspective and presented options would leak your priors and bias the answer (e.g., "what concerns you most?" where a 4-option menu signals which axes matter), or (c) you cannot write 3-4 genuinely distinct, plausibly-correct options that cover the space without padding or strawmen. The test: if you'd be straining to fill the option slots, the question is open — use prose. Rule 1 still applies: still one question per turn.
 
 ## Output Guidance
 
@@ -68,7 +71,7 @@ Before proceeding to Phase 0.2, classify whether this is a software task. The ke
 
 **Neither** (respond directly, skip all brainstorming phases) -- the input is a quick-help request, error message, factual question, or single-step task that doesn't need a brainstorm.
 
-**If non-software brainstorming is detected:** Read `references/universal-brainstorming.md` and use those facilitation principles to brainstorm with the user naturally. Do not follow the software brainstorming phases below.
+**If non-software brainstorming is detected:** Read `references/universal-brainstorming.md` and use those facilitation principles. Skip Phases 0.2–4 below — the **Core Principles and Interaction Rules above still apply unchanged**, including one-question-per-turn and the default to the platform's blocking question tool.
 
 #### 0.2 Assess Whether Brainstorming Is Needed