Add a Bifrost capture recipe for fleetdiff inspect #22

Why

A Bifrost user should be able to answer "will my traces work?" with a short local capture instead of first deploying a new metrics stack.

Small first contribution

Add examples/inspect/bifrost/README.md with the pinned version and documented tracing setup that exports OTLP to the existing file-capture collector. Reuse examples/inspect/capture.sh. Keep this to one gateway, one non-streaming synthetic request, and the default fleetdiff inspect report.

The collector repository's gateway harness has a Bifrost setup and a synthetic provider. Its Bifrost adapter currently reads local usage records rather than configuring OTLP, so the tracing configuration is the specific missing work.

Done when

• Document the actual supported tracing settings for the chosen release, with an upstream documentation link.
• Provide a no-provider-key local test using synthetic traffic and a pinned gateway image.
• Record which model, usage, user, and session attributes are present. Do not invent mappings for absent fields.
• Include the capture, inspect, and cleanup commands; preserve the existing backend and keep raw captures private and out of Git.
• Add a small capture test or synthetic fixture and link the recipe from examples/inspect/README.md.

No hosted account or paid provider should be needed for the example. If tracing in your pinned version has a limitation, describe it here before expanding the scope.