docs(planning): reword interview phrases shared with mattpocock grilling, and re-pin the interview digests #9790

Parent: #7052 (owner decision 2026-10-13: pinned planning/interview lines split into their own item). Blocked by the #7269 pin slice, because both re-pin digests in plugins/planning/tests/interview-defenses.test.sh.

Problem

These interview lines share short phrases with mattpocock/skills skills/productivity/grilling/SKILL.md, skills/productivity/grill-me/SKILL.md and skills/engineering/grill-with-docs/SKILL.md (at 24fe0ef), per the #7052 comment of 2026-10-11T16:04:

• plugins/planning/skills/interview/SKILL.md: the "too big to hold in one session" line in Stance, the "frontier is empty" phrasing in Step 2, and the composition-table line "come back and answer in one line".
• Step 3's "an empty frontier is necessary but not sufficient" (byte-pinned by interview-defenses.test.sh); reword it to the stop wording "no question is left to ask".
• context/loop.md: "question in the same round", near the branch-out mechanics.
• "the frontier is empty" in context/assumption-sweep.md (digest-pinned), templates/checklist.md, the interview evals/evals.json and /planning:wayfind.

Done when

• For each of the three upstream files, gh api -H 'Accept: application/vnd.github.raw' 'repos/mattpocock/skills/contents/<path>?ref=24fe0ef' > /tmp/grill.md then node plugins/attribution/skills/audit/scripts/fingerprint.mjs compare --local plugins/planning/skills/interview/SKILL.md --source /tmp/grill.md | jq .separation.fired prints false; same for context/loop.md.
• git grep -n -i 'frontier is empty' -- plugins/planning prints nothing.
• The byte pins and digests are updated and scripts/run-test-file.sh plugins/planning/tests/interview-defenses.test.sh passes.
• scripts/check-changelog-fragments.sh passes.