refactor(aidd-dev): redesign 01-plan around a single 02-design action

[blafourcade]

Tracking issue for in-flight work in PR #271.

Created during roadmap review 2026-06-18 to surface active framework work on the board. See PR #271 for scope and discussion.

[blafourcade]

Carve-out from #275 (now closed): the plan-ingest half.

aidd-refine:01-brainstorm now persists a refined idea with settled decisions and flagged-open assumptions to aidd_docs/brainstorm/<yyyy_mm_dd>-<slug>.md. The other half of #275 — making the plan honor those decisions — was left for the plan side.

For the 01-plan redesign, consider:

• Accept an optional brainstorm artifact (aidd_docs/brainstorm/<date>-<slug>.md) as an input alongside requirements.
• Treat its settled decisions as constraints, and surface any conflict before generating the plan, instead of silently diverging and self-correcting later.
• The brainstorm artifact also carries a flagged-open list (unresolved forks), which the plan can fold into its open questions rather than re-deciding.

Field symptom from #275: a plan that reopens decisions already locked during brainstorm, catching itself only sometimes.