docs: add 'low' reasoning level and quick validation checklist to README

[heliosran]

Motivation

• Align the README with the supported reasoning configuration by documenting the missing low effort level.
• Give operators a small set of copy/paste checks to validate the wrapper end-to-end after deployment.

Description

• Added a low effort level under the Advanced Reasoning "Effort Levels" section.
• Added a "Quick Validation Checklist" with three curl commands to verify /health, /v1/models, and a non-streaming /v1/chat/completions sanity check.
• Added brief troubleshooting guidance explaining common failures for the validation steps (deployment/network issues or 401 auth errors).

Testing

• Ran git diff --check to validate patch cleanliness and it passed.
• Confirmed repository status and that only the intended README changes are present.

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Codex Task