feat: refactor brainstorm skill into a requirements-first workflow

[tmchow]

Summary

This refactors the brainstorm skill so its durable output is a right-sized requirements document rather than a loose brainstorm note, and aligns the downstream workflow to consume that artifact consistently. This should improve the overall results, while ensuring it's right-sized for the task.

What changed

• make ce:brainstorm self-contained and remove the duplicate brainstorming helper skill
• evolve brainstorm output to docs/brainstorms/*-requirements.md with clearer product-definition guidance, adaptive ceremony, and stronger handoff rules
• keep the workflow cross-platform by using generic interactive-choice guidance instead of Claude-specific tool assumptions
• update ce:plan, document-review, and related review/protection guidance to treat the requirements doc as the upstream source artifact

Why

The old flow left a gap between brainstorming and planning: the brainstorm artifact was too loose for being the only pre-technical product-definition step, but a straight PRD port would have added too much ceremony and workflow baggage. This keeps the existing CE brainstorm momentum while making the artifact strong enough for planning to rely on without inventing product behavior.

Approach

• preserve /ce:brainstorm as the workflow name
• keep artifacts in docs/brainstorms/ to avoid churn, but use a -requirements.md suffix to clarify what the document is
• right-size the requirements doc by scope so small work stays light and larger work gets stronger structure
• keep technical implementation details out of the requirements doc unless the brainstorm itself is inherently technical
• treat unresolved questions more carefully by separating true planning blockers from items intentionally deferred to planning

Verification

• bun test