feat(pi): first-class support via pi-subagents + pi-ask-user (#651)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -30,7 +30,7 @@ This skill does not implement code. It explores, clarifies, and documents decisi
 1. **Ask one question at a time** - Do not batch several unrelated questions into one message.
 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. 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. **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.
 
 ## Output Guidance
 

plugins/compound-engineering/skills/ce-brainstorm/references/handoff.md

@@ -8,7 +8,7 @@ This content is loaded when Phase 4 begins — after the requirements document i
 
 The Phase 4 menu's visible option count varies by state: no requirements doc hides the review and Proof options, unresolved `Resolve Before Planning` hides `Plan implementation` and `Build it now`, a failing direct-to-work gate hides `Build it now`. Count the visible options for the current state and choose the rendering mode accordingly:
 
-- **4 or fewer visible:** 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). This is the default per the plugin AGENTS.md "Interactive Question Tool Design" section.
+- **4 or fewer visible:** 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)). This is the default per the plugin AGENTS.md "Interactive Question Tool Design" section.
 - **5 or more visible:** render as a numbered list in chat. This is the narrow case-3 overflow exception documented in the same AGENTS.md section; trimming would hide legitimate choices (plan, review, Proof, build, refine, pause are all distinct destinations).
 
 Never silently skip the question.

plugins/compound-engineering/skills/ce-brainstorm/references/universal-brainstorming.md

@@ -45,7 +45,7 @@ When the conversation has enough material to narrow — reflect back what you've
 
 **Always synthesize a summary in the chat.** Before offering any next steps, reflect back what emerged: key decisions, the direction chosen, open threads, and any assumptions made. This is the primary output of the brainstorm — the user should be able to read the summary and know what they landed on.
 
-**Then offer next steps** using 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. 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.
+**Then offer next steps** using 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.
 
 **Question:** "Brainstorm wrapped. What would you like to do next?"