Retire the per-turn bearer-token forwarding mechanism in favor of
transparent authentication via operator-configured headers in
fastagent.secrets.yaml. Agents now rely on long-lived team JWTs configured
per downstream MCP server.
Replace the token-forwarding patches with debug-only traceback-capture
wrappers around three opaque fast-agent catch-sites that previously
flattened exceptions to bare strings, making downstream transport errors
diagnosable.
Update README with authentication guidance and deprecation notice for
the retired `forward_inbound_auth: true` flag (now silently ignored).
