docs: add essential documents about opencode support

[swj9707]

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📝 Summary

• add essential documents about opencode support.

📄 Description

• Add minimal OpenCode quick start guidance in README.md and template TEMPLATE_README.md.
• Keep guidance minimal: launch/run examples and .env location in langgraph.json.
• Remove extra OpenCode-only docs to reduce maintenance surface.

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• uv run ruff check . passed
• uv run pytest -q passed

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