Clarify retry plugin host retry ceiling contract

[lucarlig]

Summary

The retry-with-backoff plugin still checks get_settings().max_tool_retries when that setting exists, but standalone cpex 0.1.0.dev13 does not expose max_tool_retries on cpex.framework.settings.get_settings(). In standalone CPEX, the plugin falls back to its own configured max_retries; in host environments that provide the field, it clamps to the host ceiling.

Why separate from PR #57

PR #57 is scoped to migrating plugins from gateway imports to the standalone cpex framework dependency. Changing retry ceiling policy is plugin behavior/API contract work and should be handled separately.

Acceptance criteria

• Decide whether retry ceilings are owned by the host runtime or plugin config.
• If host-owned, expose/pass the ceiling through an explicit CPEX API/config contract.
• If plugin-owned, remove the implicit max_tool_retries lookup and document the behavior.
• Add tests for the chosen contract.

[lucarlig]

Closing as out of scope for this import/dependency migration PR.