docs: restructure README for improved navigation and discoverability

- Add badges (CI, Markdown, Version, License) at top
- Add clickable table of contents for 16 sections
- Move Installation/Quick Start before Skills for better flow
- Condense 8 Skills into scannable table with backtick triggers
- Document /plugin-dev:create-marketplace workflow (was missing)
- Add Validation Agents section covering 3 agents
- Add Prerequisites and Getting Help sections
- Convert ASCII workflow diagram to Mermaid
- Streamline Use Cases into table format
- Remove "Total Content" section (hard to maintain)
- Consolidate utility scripts into single location

Reduces README from 448 to 338 lines while adding more useful content.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: sjnims