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/AGENTS.md

@@ -122,7 +122,7 @@ Keep rationale at the highest-level location that covers it; restate behavioral
 
 ### Cross-Platform User Interaction
 
-- [ ] When a skill needs to ask the user a question, instruct use of the platform's blocking question tool and name the known equivalents (`AskUserQuestion` in Claude Code, `request_user_input` in Codex, `ask_user` in Gemini)
+- [ ] When a skill needs to ask the user a question, instruct use of the platform's blocking question tool and name the known equivalents (`AskUserQuestion` in Claude Code, `request_user_input` in Codex, `ask_user` in Gemini, `ask_user` in Pi via the `pi-ask-user` extension)
 - [ ] For Claude Code, also instruct to load `AskUserQuestion` via `ToolSearch` with `select:AskUserQuestion` first if its schema isn't already loaded — `AskUserQuestion` is a deferred tool and won't be available at session start. A pending schema load is not a valid reason to fall back to text.
 - [ ] Include a fallback: when no blocking tool exists in the harness or the call errors (e.g., Codex edit modes where `request_user_input` is unavailable, or `ToolSearch` returns no match), present numbered options in chat and wait for the user's reply — never silently skip the question.
 - [ ] **Narrow exception for legitimate option overflow:** when a menu has 5 or more genuinely relevant options — each a distinct destination or workflow, none removable without losing real user choice — render as a numbered list in chat rather than trimming to fit the 4-option cap. This is used with restraint, not as a convenience escape from the blocking tool. Default remains the blocking tool. Before invoking the exception, verify that (a) no option can be cut, (b) no two options can be merged, and (c) no option is better surfaced as contextual prose (e.g., a nudge adjacent to the menu). If any of those reductions work, prefer them over the fallback. When the exception applies, include a hint that free-form input is accepted (e.g., "Pick a number or describe what you want.") so the numbered list retains the blocking tool's open-endedness.
@@ -148,7 +148,12 @@ Design rules for blocking question menus (`AskUserQuestion` / `request_user_inpu
 
 - [ ] When a skill needs to create or track tasks, describe the intent (e.g., "create a task list") and name the known equivalents (`TaskCreate`/`TaskUpdate`/`TaskList` in Claude Code, `update_plan` in Codex)
 - [ ] Do not reference `TodoWrite` or `TodoRead` — these are legacy Claude Code tools replaced by `TaskCreate`/`TaskUpdate`/`TaskList`
-- [ ] When a skill dispatches sub-agents, prefer parallel execution but include a sequential fallback for platforms that do not support parallel dispatch
+
+### Cross-Platform Sub-Agent Dispatch
+
+- [ ] When a skill dispatches sub-agents, instruct use of the platform's subagent primitive and name the known equivalents (`Agent`/`Task` in Claude Code, `spawn_agent` in Codex, `subagent` in Pi via the `pi-subagents` extension)
+- [ ] Prefer parallel execution but include a sequential fallback for platforms that do not support parallel dispatch
+- [ ] Prefer sub-agents shipped with this plugin (`ce-*`) over platform built-ins. Built-ins have different names on each target (e.g., Claude Code's `Explore` is `explorer` on Codex via `spawn_agent`'s `agent_type`, `scout` on Pi via `pi-subagents`) — using our own avoids the enumeration tax. Exception: when a built-in offers a meaningful benefit worth keeping, enumerate the per-platform equivalents inline at the call site so the model can route correctly on each target.
 
 ### Script Path References in Skills