refactor(agents): Reorganize into nested category hierarchy

Replace flat NN-category structure with nested taxonomy:
- development/{core,lang,quality,tool,experience,docs,ui,ux,release,embedded,iot,blockchain}
- infrastructure/{devops,cloud,networking,deployment}
- data/{science,bases}
- ai-ml/{model,ai,ctx,rl}
- finance, hardware, medicine (placeholders)
- info-tech/{admin,cybersec,security,pwsh}
- product/{project,research}
- business/{marketing,sales,research}
- law
- meta/{orchestration,workflows,plugins,skills}
- research/core

Also:
- Delete 126 duplicate top-level agent files (identical hashes)
- Fix dotnet-framework-48-expert and powershell-51-expert names
- Categorize 10 previously uncategorized agents
- Remove stale _context-agents symlink and .notes.md
- Remove old 01-10 category dirs with plugin.json/README files

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: aRustyDev