feat: add "ready" action - add label when all workflows green

[glasnt]

Add a new workflow for adding "Ready" when all CI is passed.

#397

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool

[glasnt]

There appears to be a fundamental issue with the intent here.

Zizmor doesn't like using using workflow_run https://docs.zizmor.sh/audits/#dangerous-triggers

Changing it to workflow_call means we have a reusable workflow, like https://github.com/beeware/.github/blob/main/.github/workflows/check-pr-template.yml, that is re-used in places like https://github.com/beeware/toga-chart/blob/main/.github/workflows/check-pr-template.yml

However, we want this check to trigger after all PR based checks, which includes multiple files (ci, check-pr-template), where we want it to run when all workflows are finished running. But we can't use workflow_run.

There might be some permutation we have not discussed, and while using https://docs.zizmor.sh/usage/#ignoring-results is an option, it's not preferrable in this setup.

[glasnt]

By all accounts, one of the only "good" ways to do this is to have all required checks live (or start) in the one workflow, then have a ready job happen after all of them, only if all checks are successful.

The alternative to all of this is to have a filter of all non-draft successful PRs in the org as the landing place for anyone looking to triage. It does work but it's not the most friendly.

(TIL "status:success" is a valid filter for PR search)