feat(planning): list prototype evidence and what stays unproven in /planning:plan #12024

Parent #7591 records the decision "prototype evidence on a throwaway branch after the secrets check" (Q167, Q168). The recommendation the owner accepted for Q168 has a second half: "/planning:plan lists prototype evidence and what stays unproven" (.work/poteto-review/deepdive/ROUND.md:303). The poteto-review ledger records the owner's answer to Q168 as "accepted as recommended audit A7" (.work/poteto-review/interview-checklist.md:213), and decision row 99 records Q160 and Q165-Q169 as "APPROVED as recommended" (.work/poteto-review/GUIDE.md:209, owner 2026-10-15). /planning:plan today consumes a /prototype:pressure-test spike's outcome (plugins/planning/skills/plan/SKILL.md:205) without listing where its evidence lives or what the spike left unproven.

Done when

• plugins/planning/skills/plan/context/plan-template.md gains a prototype-evidence section. For each prototype the plan consumes, it lists:
  • the branch and commit SHA, or the local path with not pushed: <reason>;
  • what the prototype left unproven.
• plugins/planning/skills/plan/SKILL.md's build-technique step says to fill that section from each consumed prototype's durable note. It also says to omit the section when no prototype was consumed.
• plugins/planning/evals/ gains a plan-prototype-evidence-listed case whose case.yaml uses the same schema_version as plan-brief-assumption-carried/case.yaml. Its scaffold holds a prototype note with a branch, a commit SHA and one unproven point, and its expected_outcome and graders check that the branch, the SHA and the unproven point appear in the plan's prototype-evidence section.
• A .changes/planning/*.md fragment with bump: minor describes the section.
• The PR's CI is green.

Out of scope

• Writing or pushing prototype evidence (slices S4 and S6).

Parent: #7591