refactor(aidd-refine): add the brief check and action links to the brainstorm skill #1084

Problem

aidd-refine:01-brainstorm was leaned in #1072 without two things the lean plan skill (#1080) has: a bundled check of the file it writes, and a markdown link from each action to the next. On mistral-medium-2604 a plain "go to the next action" let the model stop to ask whether to continue, and a link to the next action file removed that stop on the plan skill (4 of 4 plans written, against 2 of 3 without it).

Scope

• A markdown link at the end of capture to explore and of explore to conclude. No link back from a later action to an earlier one: on the plan skill a link back made the model loop between the two (30 judge calls on one run).
• A bundled script that checks the persisted brief against assets/discovery-brief.md (sections, Clear / Assumed / Open, status: approved, no comment or {{...}} left). It runs only when the user chose to persist the brief: conclude calls it on that branch and nowhere else.
• A test under scripts/__tests__/ with one mutation per rule, written first and seen failing, covering Windows line endings.

Acceptance criteria

• The pre-commit gates pass.
• A persisted brief that breaks a rule is reported by the script, and no brief means no script run.
• Claude haiku and mistral-medium-2604 still hold the brainstorm loop and produce the brief on "stop", on the scenarios used for refactor(aidd-refine): lean the brainstorm skill for small models #1072.

Prior art in this repo

• refactor(aidd-refine): lean the brainstorm skill for small models #1071 and refactor(aidd-refine): lean the brainstorm skill for small models #1072 (the lean brainstorm), refactor(aidd-dev): lean the plan skill for small models #1075 and refactor(aidd-dev): lean the plan skill for small models #1080 (the lean plan skill, its validate-plan.cjs and its links).

Out of scope

• plan, implement, assert, review.
• A script for a brief that was never persisted.