Update GitHub workflows: remove CompatHelper, add Julia dependabot

[Krastanov-agent]

⚠️ Important: This enables dependabot only for the top-level Julia package. If there are any other sub-packages in subfolders, they will not be tracked by dependabot and may need separate configuration.

CC @Krastanov

Summary

• Remove CompatHelper workflows (replaced by dependabot)
• Add/update dependabot.yml with Julia and GitHub Actions ecosystems
• Julia dependabot support is now stable and preferred over CompatHelper

Changes

• Deleted any CompatHelper workflow files from .github/workflows/
• Updated/created .github/dependabot.yml with Julia and GitHub Actions ecosystem configurations
• Set weekly update schedule for Julia dependencies and GitHub Actions

Test plan

• Verify no CompatHelper workflows remain in .github/workflows/
• Confirm dependabot.yml has proper Julia and GitHub Actions ecosystem configurations
• Check that dependabot starts working after merge (may take up to 24 hours)

🤖 Generated with Claude Code

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Script source: https://gist.github.com/Krastanov-agent/2b049684a01788458c399e9441bb264a

[Krastanov]

This PR should simplify the handling of compat updates, now that Julia is supported by dependabot. CompatHelper is not needed anymore. And dependabot does not stop after a few months of inactivity in the repository. There are a bunch of other minor quality of life improvements as well.

This PR was made by a bot, but now I am commenting as a human check on the bot that I endorse this PR and have checked its correctness.

@simsurace , could you vet this one -- it should be pretty quick. I want to avoid merging things unilaterally when there are other volunteers who have interacted with the codebase recently.

[gdalle]

Seems fine to me (I am also adding dependabot to all of my repos). Here we could add the "/docs" directory too though

[Krastanov]

The docs folder is included now. In most of my packages I do not have any compat bounds on the docs folder -- I think of it as another compat CI test. That is why I did not think to include it here. I kinda prefer not having these compat bounds

[gdalle]

Honestly I don't think it matters much either way. Compat bounds in docs and tests are most useful for packages with many dependencies or frequent updates, but here we're not in either of those cases