feat(prototype): add an observe option to /prototype:pressure-test #12022

Parent #7591 decides that "/prototype:pressure-test gains observe" (Q167), but its lane run stopped with reason intent (comment 6556369883) because the issue did not say what observe observes or produces; the poteto-review interview answered it. The poteto-review ledger records the owner's answer to Q167 as "accepted as recommended audit A7" (.work/poteto-review/interview-checklist.md:212), and decision row 99 records Q160 and Q165-Q169 as "APPROVED as recommended" (.work/poteto-review/GUIDE.md:209, owner 2026-10-15). The session's recommendation the owner accepted (.work/poteto-review/deepdive/ROUND.md:292-294), verbatim: "Should /prototype:pressure-test gain an observe option where the agent runs the variants of a fork a run can settle, records behavior, output and timing (timing labeled unqualified), and recommends, handing off to /performance:goal only when the fork is itself a performance target?" with the recommended answer "Yes, as described; the human keeps product and preference calls; the STOP gate for unrequested prototypes stays." The problem it solves is that "questions a quick run could settle reach the person", while pressure-test today is driven by a person's key presses.

Done when

• plugins/prototype/skills/pressure-test/SKILL.md names observe in its argument-hint and has a section for it. The section says:
  • the agent builds and runs each variant of a fork whose answer a run can settle;
  • it records behavior, output and timing for each variant, with timing labeled unqualified;
  • it ends with a recommendation naming the variant and the observed evidence.
• That section says that product and preference calls stay with the person, and that it hands off to /performance:goal only when the fork is itself a performance target.
• The STOP gate for an unrequested prototype is unchanged: the diff edits no line of plugins/prototype/context/discipline.md. observe keeps the existing durable-note capture and deletes the shell when done.
• plugins/prototype/evals/ gains a case whose case.yaml meets the following:
  • it uses the same schema_version as pt-html-demo-offline/case.yaml;
  • its prompt invokes /prototype:pressure-test observe on a fork two variants can settle;
  • its expected_outcome states the per-variant record, the unqualified-timing label and the recommendation.
• plugins/prototype/README.md lists the option, and a .changes/prototype/*.md fragment with bump: minor describes it.
• The PR's CI is green.

Out of scope

• Keeping prototype evidence on a throwaway branch (Q168): slices S4 to S6.
• Qualified timing, which belongs to /performance:snapshot.
• A third prototype skill (rejected in the decision).

Related

• Concise skills rollout: prototype (2 skills, 2 model-invoked) #6589 rewrites the prototype skills.

Parent: #7591