brainstorm

The user wants to brainstorm about: $ARGUMENTS

Determine the topic slug (short kebab-case, 3-5 words max).

Check if work/<slug>/ already exists (the topic may already have artifacts).
- If it exists, check for an existing brainstorm.txt. If one exists, read it
  and let the user know what's already there before continuing.
- If work/<slug>/ doesn't exist, create it.

Run the brainstorm as a conversation. Ask the user what they're thinking about,
then help them generate ideas. Keep the energy divergent - don't narrow down or
evaluate yet. Just get ideas out.

When the session feels complete (or the user says they're done), write the file to:
  work/<slug>/brainstorm.txt

File structure:

```
BRAINSTORM: <topic>
DATE: <date>

## Problem / Opportunity

<1-3 sentences framing what this is about>

## Ideas

- <idea 1>
- <idea 2>
- ...

## Open Questions

- <question 1>
- <question 2>
- ...

## Constraints / Evaluation Criteria

- <anything mentioned that would help filter ideas later>
```

IMPORTANT:
- List open questions but do NOT walk the user through them one by one.
  The point is capture, not convergence. The user will revisit when ready.
- Keep ideas as a flat list. No ranking, no grouping, no filtering.
- If the user mentions constraints or criteria, capture them. Don't prompt for them.