Add OpenAIProvider and AGENTS.md; default to OpenAI for Codex runs

[tuhinkanti]

Motivation

• Make the project runnable with the fewest code changes for Codex users by preferring OpenAI as the default provider.
• Provide a minimal, explicit run guide and a low-LOC change policy so subsequent edits stay small and deterministic.

Description

• Add AGENTS.md with a Codex-focused quick run and change policy for low-friction runs using OPENAI_API_KEY.
• Add OpenAIProvider (src/main/java/ai/openclaw/agent/OpenAIProvider.java) implementing LlmProvider using the OpenAI Chat Completions API for simple text completions.
• Wire provider selection in the gateway via a new createProvider helper in GatewayCommand so anthropic remains supported but the default is openai.
• Change config defaults in OpenClawConfig to provider = "openai" and model = "gpt-4o-mini", and update ConfigLoader to prefer OPENAI_API_KEY with a fallback to ANTHROPIC_API_KEY.
• Update README.md examples and Docker run instructions to reflect the OpenAI-first defaults.

Testing

• Ran ./gradlew build to validate the build, which failed in this environment due to a missing Java 21 toolchain (toolchain auto-provisioning not configured).

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Codex Task