triage: label no_autobump-only PRs

[GunniBusch]

• Have you followed the guidelines for contributing?
• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?

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• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes.

AI usage: drafted and iterated the triage label rules and test setup.
Manual verification: ran actionlint .github/workflows/triage.yml, inspected PR file patches, and validated outcomes using fork test PRs.

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Update triage labeling so PRs that only modify no_autobump! because: ... lines in formula files get CI-syntax-only and autobump.

Validation PRs on fork:

• GunniBusch/homebrew-core#2 (invalid mixed test: workflow + formula)
• GunniBusch/homebrew-core#3 (single formula no_autobump line change)
• GunniBusch/homebrew-core#4 (multiple formulae, only no_autobump line changes)
• GunniBusch/homebrew-core#5 (multiple no_autobump changes plus one extra non-no_autobump formula change)
• GunniBusch/homebrew-core#6 (remove no_autobump stanza with adjacent blank-line deletion)

Close

closes GunniBusch#2
closes GunniBusch#3
closes GunniBusch#4
closes GunniBusch#5
closes GunniBusch#6