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Introduce endpoint.go with shared constants, NormalizeInboundEndpoint, DeriveUpstreamEndpoint, InboundEndpointMiddleware, and context helpers. This replaces the two separate normalization implementations (OpenAI and Gateway) with a single source of truth. Includes comprehensive test coverage.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode

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Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>