Enable static request headers for Cowork custom connectors

### Problem Statement

Surface: Claude Cowork

The Add custom connector dialog exposes only OAuth Client ID and OAuth
Client Secret. It does not expose the Request headers section documented
for the static_headers beta.

My trusted personal remote MCP server is reachable and authenticates using
the x-brain-key request header. The same endpoint is working successfully
from Claude Code using header authentication.

### Proposed Solution

Request:
1. Enable the static_headers beta for my account/Cowork surface.
2. Confirm whether x-brain-key is permitted by the request-header allowlist,
or add it to the allowlist.

I do not want to place the credential in the URL, remove authentication, or
introduce an unnecessary proxy. It's early in the project and I'd rather not move to full OAuth yet if I can avoid it. Thanks in advance!

### Alternatives Considered

_No response_

### Area

OAuth / Authentication

### Priority

Medium - Would be helpful

### Additional Context

_No response_