Move tool metadata out of MCPToolRegistry

[Gabriel-Darbord]

Summary

• make MCPToolRegistry an instance-scoped catalog with no class-side methods or global registration state
• derive built-in registrations from concrete MCP tools instead of hardcoded class lists
• move groups, keywords, examples, activation requirements, default exposure, mutability, and risk onto tool instances and their hierarchies
• keep user visibility overrides in registry-aware MCPToolExposurePolicy instances
• make singular query normalization fully rule-based
• expose tool-owned catalog metadata through tool_get while keeping tool_search metadata compact
• make plugin registrations isolated per registry

Verification

• P13 focused registry/catalog tests: 137 passed
• P13 MCP-Tests and MCP-UI-Tests: 723 passed
• no built-in tools fall into the default other group
• MCPToolRegistry has 0 class-side methods
• git diff --check