Fix image provider selection flow

[laiyangjun-dotcom]

Summary

• Require users to choose a fixed image provider (gemini-native or openai-images) during setup instead of auto-switching at runtime.
• Add provider-isolated key/model resolution: Gemini uses GEMINI_API_KEY; OpenAI-compatible uses IMAGE_GEN_API_KEY / OPENAI_API_KEY.
• Update image generation, preflight, install flow, runtime template, and skill docs to reflect the fixed-provider workflow.

Validation

• python3 -m py_compile scripts/image.py scripts/preflight.py
• bash -n install.sh
• Provider isolation smoke test via direct scripts/image.py import
• IMAGE_GEN_PROTOCOL=auto IMAGE_GEN_API_KEY=dummy python3 scripts/image.py --check exits 2
• IMAGE_GEN_PROTOCOL=gemini-native GEMINI_API_KEY=dummy python3 scripts/image.py --check exits 0
• IMAGE_GEN_PROTOCOL=openai-images IMAGE_GEN_API_KEY=dummy python3 scripts/image.py --check exits 0

Notes

• Runtime secrets are intentionally excluded. config/runtime.env is not included; only config/runtime.env.example is updated.