docs: add project architecture and planning documents

context/openai-context.md

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+# OpenAI Integration
+
+Provider:
+
+OpenAI
+
+Interface:
+
+Responses API
+
+Model Selection:
+
+Configurable
+
+Default Behaviour:
+
+- Low temperature
+- Deterministic outputs
+- Structured responses
+
+Requirements:
+
+- API key via environment variable
+- Rate limit awareness
+- Error handling
+- Retry logic