An OAuth client on /mcp is capped by the access it was approved with.
When another environment calls on behalf of one of its agents, that agent
may run under narrower modes, and the target has no way to know.

/mcp now reads `T3-Mode-Limit: <runtimeMode>/<interactionMode>` from OAuth
clients and caps the request at the narrower of the two. The header can
only narrow: a broader one changes nothing, and a malformed one is
refused with 400. The client's interaction mode can now be capped too;
before, it was always default.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>