Prove /setup-pstack from a T3 Code session #280

Current scope

#253's acceptance 2 ("/setup-pstack works from a T3 session") shipped as instructions in PR #272 (1.19.0) but was never run live: the T3 live-gate recipe covers only t3:dispatch, and setup could not run on studio1 then because Grok's sandbox failed (fixed in #275).

Acceptance:

1. A t3:setup live case (or the smallest extension of t3:dispatch) runs installed /setup-pstack in a run-owned disposable T3 server with a fixture model sheet, and asserts from the transcript: T3-routed families probed through delegate_task, runner-routed families probed through the runner, the sheet written only after every probe passes, and the operator's real config untouched.
2. Live gate: that one case plus the mandatory --self-test, on studio1 (the only host with the T3 service).

Source: batch 262 closeout; PR #272.