Add support for custom OTEL resource attributes in config

What feature would you like to see?

We need to pass custom resource attributes (e.g. team, department, cost_center) via OTEL config for usage tracking and cost allocation.

Use case: We're running Codex across multiple teams, environments with different billing models - some token-based, some seat-based. We're already collecting telemetry via OTEL but the built-in attributes aren't enough to segment usage properly. Being able to inject custom resource attributes would let us tag sessions with team, project, environment, etc. for accurate cost allocation and usage dashboards.

Proposed config:

[otel.resource_attributes]
team = "platform"
department = "engineering"

Would be happy to open a PR if this approach works.

Additional information

_No response_