feat: add Extension Store with plugin catalog and MCP registry
Full Extension Store implementation (Phases 0-6):
- Plugin marketplace catalog with ETag caching and search/filter/sort
- MCP server registry with Official + Glama aggregation
- Install/uninstall flows for both plugins and MCP servers via CLI
- Per-tab UI state, skeleton loading, dashed empty states, card polish
- Input validation and security hardening (scope allowlists, env/header
key regex, projectPath validation, HTTP body size limits)
- 8 test suites covering catalog, install, aggregation, normalizers
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