Add migration guide: from raw OpenAI SDK to JamJet

[sunilp]

We have migration guides for LangGraph and CrewAI, but the most common starting point is raw openai SDK calls.

Add a guide showing:

1. A typical raw OpenAI SDK pattern (sequential LLM calls, manual state passing)
2. The equivalent JamJet workflow
3. What you gain: execution timeline, crash recovery, eval harness, MCP tools

Where: jamjet.dev repo at src/content/docs/migrate/from-openai-direct.md — follow the same structure as existing migration guides.

Good first issue? Yes — documentation only, no code changes. Great if you have used the OpenAI SDK and want to explain the mental model shift.