The shared MCP_TOKEN guard moved from 'Authorization: Bearer <MCP_TOKEN>' to the
X-MCP-Token header (Authorization is now per-user Basic/Bearer), silently breaking
existing /mcp clients. Document it as a Breaking Change in CHANGELOG (reconfigure
to X-MCP-Token). Add a once-per-process migration warning: when MCP_TOKEN is set,
no x-mcp-token is present, and Authorization carries the old 'Bearer <MCP_TOKEN>',
log a hint to migrate — without changing the auth decision (still rejected) or
logging the token value.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
