feat(pi): support pi reviewer model routing

pi-package/skills/create-plan/SKILL.md

@@ -71,17 +71,30 @@ If any dependency is missing, stop and return:
 
 If the user already specified a reviewer CLI and model, use those values. Otherwise ask:
 
+Reviewer CLI: `codex`, `claude`, `cursor`, `opencode`, `pi`, or `skip`
+
 1. Which CLI should review the plan?
    - `codex`
    - `claude`
    - `cursor`
    - `opencode`
+   - `pi`
    - `skip`
 2. Which model?
 3. Max review rounds? Default: `10`
 
 Store `REVIEWER_CLI`, `REVIEWER_MODEL`, and `MAX_ROUNDS` for the review loop.
 
+When `REVIEWER_CLI=pi`, the reviewer model is configured independently from the pi model running this workflow. Use any configured pi model string, including provider-qualified model IDs. If the reviewer model or provider is unavailable, surface the review helper stderr/status and ask for a configured model; use `pi --list-models [search]` to inspect configured models.
+
+The pi reviewer command rendered into `/tmp/plan-review-${REVIEW_ID}.sh` must be isolated and read-only:
+
+```bash
+pi --no-session --no-skills --no-prompt-templates --no-extensions --no-context-files --model "$REVIEWER_MODEL" --tools read,grep,find,ls -p "Read the file /tmp/plan-${REVIEW_ID}.md and review."
+```
+
+The pi reviewer invocation must not load workflow skills and must not include `write`, `edit`, or `bash` tools.
+
 ### Phase 4: Design
 
 - Load `brainstorming`.