CI: resolve latest haybarn at runtime; add Dependabot + auto-merge #7

Job	Run time
resolve-haybarn	4s
unit (macos-latest)	12s
unit (windows-latest)	32s
unit (ubuntu-latest)	14s
integration (ubuntu-latest, haybarn_unittest-linux-amd64.zip)	14s
integration (macos-latest, haybarn_unittest-osx-arm64.zip)	17s
integration (windows-latest, haybarn_unittest-windows-amd64.zip)	41s
	2m 14s

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