docs(.env.example): add HF_BASE_URL placeholder

[BBCrypto-web]

What does this PR do?

Adds the missing commented HF_BASE_URL placeholder to .env.example.

Every first-class provider section in .env.example documents its *_BASE_URL override except Hugging Face. HF_BASE_URL is a supported override checked by Hermes provider/config/doctor code and defaults to:

https://router.huggingface.co/v1

This PR adds the missing commented placeholder for consistency.

Related Issue

N/A

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• 🌟 New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• Added a commented HF_BASE_URL placeholder to .env.example
• No code changes
• No runtime behavior changes
• No config behavior changes
• No unrelated provider keys changed

Notes

This is independent of #28722, which only relocates the OPENCODE_GO_BASE_URL line.

[teknium1]

Merged via PR #49944 (docs salvage cluster 2). Your commit was cherry-picked onto current main with your authorship preserved in git log — rebase-merged so it lands under your name. Thanks for the contribution!