chore: consolidate product-management brainstorm command into skill

PR #93 added brainstorming as a command + reference skill after the
commands-to-skills migration (2d6f7e2). Consolidate into a single
self-contained skill to match the current convention:

- Merge command workflow (steps, connectors, follow-up) with skill
  reference material (modes, frameworks, anti-patterns)
- Rename skill from product-brainstorming to brainstorm to follow
  the short action-oriented naming convention used by other skills
- Add argument-hint, Usage section, and CONNECTORS.md reference
- Use imperative intro style instead of persona framing
- Remove now-empty commands/ directory

Author: johnw424