#8575: feat: add HEYGEN_API_KEY to expected shell environment variables

## Summary Adds `HEYGEN_API_KEY` to the list of expected shell environment variables that OpenClaw auto-imports during onboarding. HeyGen builds the world's leading avatars and video agent. Users who want avatars for their claws or have the ability for their claws to generate videos would greatly benefit from having this integration. This allows users who have their HeyGen API key set in their shell profile (`.bashrc`, `.zshrc`, etc.) to have it automatically detected and imported, similar to ElevenLabs and other provider API keys. ## Changes - Added `HEYGEN_API_KEY` to `SHELL_ENV_EXPECTED_KEYS` in `src/config/io.ts` ## Test plan - [x] Verified the key is added in the correct location (after `ELEVENLABS_API_KEY`) - [x] Manual test: Set `HEYGEN_API_KEY` in shell profile, run onboarding, verify it's imported --- 🤖 Generated with [Claude Code](https://claude.com/claude-code) <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Adds `HEYGEN_API_KEY` to `SHELL_ENV_EXPECTED_KEYS` in `src/config/io.ts`, which is passed into `loadShellEnvFallback(...)` during onboarding/config loading. This lets OpenClaw automatically detect/import a HeyGen key from the user’s shell environment alongside other provider keys (e.g., ElevenLabs) when shell env fallback is enabled. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - Change is a single additive constant entry in an existing allowlist used for shell env import; no control flow or parsing behavior changes, and the new key matches the established naming pattern. - No files require special attention <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->