docs: sync MCP tool prefix examples

[Osraka]

Summary

• Update the MCP provider docs to use the current double-underscore tool prefix format.
• Show normalized server names in the discovery example so it matches the implementation in src/mcp/connection.ts and src/hooks/useAgentLoop.ts.
• Add a short note explaining why dots are avoided in provider-facing tool names.

Validation

• git diff --check
• Documentation-only change; no runtime tests run.