Remove TokenCountingGateway (token counting embedded in agn)

[rowan-stein]

User request

Per agynio/architecture (commit ee8a07e), token counting is embedded inside agn and is no longer a standalone service.

Specification (research)

Remove the token-counting Gateway surface area:

• Stop registering TokenCountingGateway handlers.
• Remove upstream client plumbing + config:
  • TOKEN_COUNTING_GRPC_TARGET
  • any tokenCountingClient fields
• Remove token-counting proto generation paths in build scripts/Dockerfile.
• Update Helm chart env wiring (TOKEN_COUNTING_GRPC_TARGET).

Outcome:

• Gateway no longer exposes any token-counting endpoint.