Orchestrate: restore the model-family verifier rule and the path for runtimes without nested agents #265

Current scope

Two defects the 0.15.36 sync introduced in plugins/pstack/skills/poteto-mode/playbooks/orchestrate.md:

1. Line 13 says each PR's verifier is "on a different provider". Upstream says "on a different model family" (cursor/main:pstack/skills/poteto-mode/playbooks/orchestrate.md:13). With a native-only sheet, such as Claude Opus and Sonnet, no different provider exists, so the required verifier pass is unattainable. Found by Unfret on sync: Cursor pstack 0.15.36 as Open Pstack 1.17.0 (#262) #264 (v1:89d286e5…).
2. Line 11 assigns workers to a sub-coordinator, but on Claude Code a subagent cannot start agents (pstack's native lanes deny Agent and Task). The 1.16.0 playbook said a sub-coordinator spawns its workers "where the runtime lets a subagent spawn one" and gave the other path; the rewrite dropped it. Found by Greptile on sync: Cursor pstack 0.15.36 as Open Pstack 1.17.0 (#262) #264.

Acceptance:

• The verifier rule says a different model family, as upstream does.
• Where a subagent cannot start agents, the coordinator starts the sub-coordinator's workers; the sub-coordinator writes their kickoffs and reads their results. Put the harness fact in codex-tools.md if it is not already there, per AGENTS.md's mapping-boundary rule.
• Gates pass; live gate per verify-open-pstack.

Introduced by: #264