auth: tenant-aware remote header resolution is never wired (tenant_id is always None) #353

Found by the 2.8.0 docs audit.

What

resolve_remote_headers_for_tenant (src/pmcp/remote_auth.py:121) reads tenant-isolated credentials from .pmcp/tenants/<id>/pmcp.env when given a tenant_id, and _remote_headers in src/pmcp/client/manager.py (~950) accepts tenant_id. But its only callers (client/manager.py:2578 and :2594) never pass one, so the tenant path is dead code and every remote connection resolves headers from the process environment. The README described tenant-aware header resolution as live; the 2.8.0 docs audit corrects the README to say it is not wired yet.

To decide

Where a tenant id comes from for a connection (per-request context, per-server config, or a gateway-level tenant), and how it reaches the connect path; or remove the unused path. Tests: a remote server's headers resolve from the tenant file when a tenant is set, and never from another tenant's file.

See specs/tenant-code-mode-host-contract.md.