#12388: docs: improve .env.example with comprehensive configuration examples

This PR significantly improves the `.env.example` file by adding comprehensive configuration examples for all supported environment variables. ## Changes - **Channel Tokens**: Added examples for Telegram, Discord, Slack, WhatsApp (Twilio), Signal, and Google Chat - **Model API Keys**: Added sections for OpenAI, Anthropic, and Gemini - **Optional Configuration**: Documented gateway port, profile, state directory, and log level options - **Development/Testing**: Added commonly used development environment variables - **Documentation**: Added inline comments describing each variable and where to obtain tokens/keys - **Security Notice**: Added warning about keeping API keys secure and never committing .env files ## Impact This helps new developers set up OpenClaw faster by providing a complete reference for all environment variables in one place, rather than hunting through documentation. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Updates `.env.example` from a Twilio-only stub into a comprehensive environment-variable reference, adding commented examples for multiple chat channels (Telegram/Discord/Slack/Twilio WhatsApp/Signal/Google Chat), model provider API keys (OpenAI/Anthropic/Gemini), and optional runtime/dev toggles (port/profile/state dir/log level/skip channels/live test), plus a security warning about not committing secrets. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge after a small formatting fix. - Changes are documentation-only and limited to `.env.example`; the only concrete issue found is the missing trailing newline at EOF, which should be fixed to avoid persistent diff noise and tooling inconsistencies. - .env.example (add trailing newline) <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->