Run Hub workflows when a pull request head changes

[thaning0]

Workflow

I use a Hub workflow to act on pull requests when they are opened. After review feedback, contributors can push more commits or otherwise change the head of the same pull request. I want Hub to run a follow-up workflow against the new head so the automation works with the current code.

Current limitation

Hub has semantic GitHub triggers for pull request creation, comments, and labels, but not for changes to an existing pull request head. github.pull_request_created covers only the initial opening, so later head updates cannot be expressed through the semantic trigger set.

Desired flow

1. A pull request is opened and the initial workflow runs.
2. A contributor updates the pull request head.
3. GitHub sends a pull_request event with the synchronize action.
4. Hub starts the configured follow-up workflow with the updated pull request context.
5. Existing repository, sender, and other GitHub filters apply normally.

Possible direction

Add a semantic event such as github.pull_request_updated, classified only from pull_request.synchronize. Metadata-only actions such as edited would not match it, and existing raw and semantic GitHub triggers would keep their current behavior.

The public name is intentionally open for discussion: pull_request_updated is approachable, while a name such as pull_request_head_updated would describe the boundary more precisely.

I have a focused Hub implementation and automated coverage ready. I can open the Hub implementation PR and companion public-docs PR once the event semantics and name are agreed.

Would this workflow fit Hub, and which semantic event name would be preferred?

[thaning0]

getpaseo/hub#79