ci: add PR explainer workflow (cloud + --upsert)

[utsengar]

Summary

Dogfoods @htmlbin/cli on this repo's own PRs. On every push, a coding agent reads the diff and writes a self-contained HTML explainer to ./preview.html; the CLI publishes it via the cloud backend with --upsert --metadata repo=... --metadata pr=..., so the URL is stable for the PR's lifetime. A sticky comment carries the URL and updates in place on each push; the drop is deleted when the PR closes.

Combines the two existing example workflows:

• examples/agent-preview-workflow.yml — agent-generated HTML
• examples/cloud-upsert-workflow.yml — cloud backend + metadata-keyed upsert

Required secrets (one-time setup)

Before this workflow does anything useful, add two repo secrets:

• HTMLBIN_TOKEN — run htmlbin login locally, then cat ~/.config/htmlbin/token and paste it
• ANTHROPIC_API_KEY — for Claude Code, which writes the explainer HTML

gh secret set HTMLBIN_TOKEN     --body "$(cat ~/.config/htmlbin/token)"
gh secret set ANTHROPIC_API_KEY --body "sk-ant-..."

Behavior

• opened / synchronize / reopened → publish job runs, sticky comment shows the explainer URL
• closed → teardown job deletes the drop and updates the sticky comment
• Forked-PR runs won't have the secrets and will fail at the explainer step (standard GitHub Actions behavior for pull_request)

Test plan

• Add HTMLBIN_TOKEN and ANTHROPIC_API_KEY repo secrets
• Merge this PR (the workflow can't fire on the PR that introduces it)
• Open a follow-up PR; confirm sticky comment appears with a working URL
• Push a second commit to that PR; confirm same URL, refreshed page
• Close the PR; confirm drop is deleted and sticky comment updates

🤖 Generated with Claude Code

[github-actions]

PR explainer: https://htmlbin.dev/p/r6w2pQhgT

Generated from the diff against main (12 KB). Updates on every push; deleted when this PR closes.