Add copilot env sample and seeding

copilot.env.example

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+# AFFiNE Copilot sample environment overrides
+# Copy this file to .env (or use Cloudron's env UI) and adjust values.
+
+# Toggle Copilot globally (true/false).
+AFFINE_COPILOT_ENABLED=true
+
+# OpenAI-compatible provider (Official OpenAI, OpenRouter, local proxy, etc.).
+AFFINE_COPILOT_OPENAI_API_KEY=sk-your-openai-key
+AFFINE_COPILOT_OPENAI_BASE_URL=https://api.openai.com/v1
+
+# Anthropic Claude provider.
+AFFINE_COPILOT_ANTHROPIC_API_KEY=sk-ant-your-key
+AFFINE_COPILOT_ANTHROPIC_BASE_URL=https://api.anthropic.com/v1
+
+# Google Gemini provider (API key or reverse proxy).
+AFFINE_COPILOT_GEMINI_API_KEY=your-gemini-key
+AFFINE_COPILOT_GEMINI_BASE_URL=https://generativelanguage.googleapis.com/v1beta
+
+# Optional Exa web search key for Copilot browsing.
+AFFINE_COPILOT_EXA_KEY=exa-key
+
+# Override the scenario/model mapping by providing JSON.
+# Example keeps override_enabled true and customizes a few models.
+AFFINE_COPILOT_SCENARIOS_JSON={"override_enabled":true,"scenarios":{"chat":"gpt-4o-mini","coding":"claude-3-5-sonnet-20241022","embedding":"text-embedding-3-large","quick_text_generation":"gpt-4o-mini"}}