Introduce `model_capabilities.mantle` flag that installs a provider-specific
override in fast-agent's `ModelDatabase._PROVIDER_MODEL_OVERRIDES` to strip
features the AWS Bedrock Mantle endpoint rejects (beta headers, extended
thinking, task budgets, web tools, prompt caching).
Without this override, fast-agent sends default beta headers and `thinking`
parameters for modern Claude models that Mantle rejects with a misleading
404 "model does not exist" error.
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).
