planning:interview: support ad hoc interviews without writing a PLAN.md Brief

[kyle-sexton]

Problem

An engineering-domain /planning:interview always writes a ## Brief into docs/topics/<topic>/PLAN.md on the task branch. For small ad hoc interviews, such as a scope check inside a larger workflow (one question round before a skill run), that commits a planning file per use. contract_tier: local avoids the commit but still produces the full Brief and ledger.

Surfaced while designing a repo-sweep playbook, where every step asks a short scope round before running a hygiene skill; a Brief per step would add a planning file per step to each PR.

Proposal

A lightweight mode (argument or auto-detected) that runs the rounds and writes no Brief when the caller only needs answers:

• The open-question ledger is still written to the memory slice, and the register gate still applies when questions were asked.
• The decisions are returned to the caller, which records them in its own durable artifact (for example, a Scope decisions: section in the commit the answers shaped).

Acceptance

• A caller can invoke /planning:interview for a short scope round and get decisions back with no PLAN.md written or committed.
• The ledger and register gate behave as today.

[kyle-sexton]

From the 2026-09-29 audit of the Cursor run. Reopened: an owner decision is needed. Labels: needs-triage removed, needs-human and status: needs-decision added.

Question. Keep the interview scope action that #4889 added, and if kept, should repo-sweep call it?

Facts.

• The operator filed this issue for a repo-sweep step.
• plugins/playbooks/skills/repo-sweep/reference/next.md:85 (committed 2026-09-27 in 6cc3a12/fix(playbooks): bind repo-sweep next to skill deliverables and synced files (#4602 #4603) #4627, before feat(planning): interview scope action without PLAN.md Brief (#4502) #4889 merged) says "Do not run /planning:interview for this. Record each question and answer for the commit."
• After feat(planning): interview scope action without PLAN.md Brief (#4502) #4889 there was no in-repo caller and no eval. Draft PR fix(planning): audit remediation (interview scope, outcome gate, plan-reviewer, evals, re-attach slice) #5330 adds an eval and fixes the dropped deferred/blocked rows.
• The action adds about 1.2 KB to a 65 KB hub and a fourth persist path, and forced five digest re-pins.
• docs/plugin-philosophy.md:852 ("Instruction economy") asks for observed-stumble evidence before standing additions.

Options.

• A: keep scope and edit repo-sweep next.md:85 to call /planning:interview scope (a playbooks change: one line plus its eval).
• B: keep scope unwired until a caller exists (this branch's state).
• C: revert feat(planning): interview scope action without PLAN.md Brief (#4502) #4889's scope action (restore the pre-feat(planning): interview scope action without PLAN.md Brief (#4502) #4889 interview text, re-pin digests, drop the 0.45.5 mention).

Recommendation: A. Basis: the operator's own issue names repo-sweep as the caller and next.md:85 predates the action, so the contradiction reads as a stale line rather than a rejection (file:line above); judgment on the hub cost.

Unblocks: the playbooks edit (A) or the revert (C).

PR #5330 uses Refs #4502, never Closes.