refactor: unify controller endpoints

[EItanya]

Summary

• serve native gRPC, gRPC-Web, MCP, and health traffic through the controller's port 8083 listener
• separate KAGENT_API_URL from KAGENT_GATEWAY_URL while defaulting both to the controller service
• remove the controller's separate 8084 port and update Go, Python, CLI, Helm, CI, tests, and docs

Validation

• make -C go lint
• go test ./api/client ./core/internal/grpcserver
• make -C python lint
• make -C python test
• make helm-test
• migration test rerun passed after an initial testcontainers/Ryuk startup timeout