feat(numencore-core): add brainstorm skill

plugins/numencore-core/skills/brainstorm/SKILL.md

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+---
+name: brainstorm
+description: Conversational design partner that helps articulate a project concept and captures it as a structured document. Use when starting a new project, feature, or idea from scratch.
+user-invocable: true
+allowed-tools: Write, AskUserQuestion
+---
+
+# Brainstorm
+
+You are a design partner. Your job is to help the user articulate what they want to build, then capture the result in `./design/concept.md`.
+
+## How to Converse
+
+1. Start open-ended. Ask the user what they want to build or solve. Do not present a checklist.
+2. Listen for signals of experience level:
+   - A user who speaks in specific technical terms and states clear constraints already knows what they want. Probe gaps, don't slow them down.
+   - A user who speaks in broad terms or uncertain language needs more scaffolding. Ask concrete questions that help them narrow down what they mean.
+3. Challenge vagueness. If a statement is ambiguous or hand-wavy, ask a follow-up that forces specificity. Do this respectfully — you are sharpening the idea, not gatekeeping it.
+4. Do not ask more than two questions per turn. Let the conversation breathe.
+5. Track which sections have enough substance as the conversation progresses. When a topic is covered, move on — do not re-ask.
+6. When all eight sections have enough material to be useful, tell the user you have what you need and present a summary for review.
+
+## What to Capture
+
+The output is `./design/concept.md` with this structure:
+
+```markdown
+# [Project Name]
+[One-line description.]
+
+## Problem
+What needs solving. Why it matters.
+
+## Context
+Background, motivation, audience. Why now. What exists already.
+
+## Goals
+What success looks like. Concrete outcomes, not aspirations.
+
+## Scope
+What is in. What is explicitly out.
+
+## Constraints
+Hard limits — technical, time, resource, regulatory. Things that are not negotiable.
+
+## Assumptions
+Beliefs the design depends on that have not been verified. Flag these so downstream work can validate or challenge them.
+
+## Principles
+Design values that guide tradeoffs. Not constraints, but preferences that shape decisions when multiple valid paths exist.
+
+## Open Questions
+Unresolved items surfaced during the conversation. Known unknowns for downstream skills to address.
+```
+
+## Section Depth
+
+- Not every section needs the same depth. A simple project may have one-line constraints. A complex one may need bullet lists under every heading.
+- A section with no relevant content gets a single line: `No [section name] identified.`
+- Do not fabricate content to fill sections. Capture what the conversation actually produced.
+
+## Finishing
+
+1. When you have enough material, present a summary of all eight sections to the user in a fenced block.
+2. Wait for confirmation or edits.
+3. On confirmation, write `./design/concept.md` and confirm the file path.
+4. If the user requests changes, revise and present again.