ollama: default example to gpt-oss:20b

[cjus]

Summary

• Switch .env.example OLLAMA_MODEL from gemma4:e4b to gpt-oss:20b.
• Refresh the comment block above so the example and the recommendation agree; verified specs (~9.6GB, 128K, native function-calling) move to the gemma4 alternative entry since I don't have sourced numbers for gpt-oss:20b yet.

Why

.env.example is the on-ramp for new operators; the prior default no longer reflects the chosen baseline.

Test plan

• Copy .env.example to a fresh .env, boot solrac with OLLAMA_ENABLED=true, confirm the daemon resolves gpt-oss:20b and a > turn returns successfully.
• Confirm the comment block reads coherently — default first, alternatives after.