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
- Update README.md skills table to reflect rename
- Remove now-empty commands/ directory

Author: johnw424

product-management/README.md

@@ -42,7 +42,7 @@ This plugin gives you an AI-powered product management partner that can help wit
 | `user-research-synthesis` | Thematic analysis, affinity mapping, persona development, opportunity sizing |
 | `competitive-analysis` | Feature comparison matrices, positioning analysis, win/loss analysis |
 | `metrics-tracking` | Product metrics hierarchy, goal setting (OKRs), dashboard design, review cadences |
-| `product-brainstorming` | Brainstorming modes (problem exploration, solution ideation, assumption testing, strategy), PM frameworks, session structure, thinking partner behaviors |
+| `brainstorm` | Brainstorming modes (problem exploration, solution ideation, assumption testing, strategy), PM frameworks, session structure, thinking partner behaviors |
 
 ## Example Workflows