fix: replace raw echo with standard output helpers in github/cancel-workflows

[dnyw4l3n13]

Summary

• Replace the custom inline info() and die() functions with the standard project output helpers (die, info, success), using coloured printf-based output per shell-scripts.examples.md
• Add success call at end of script to confirm completion
• Change shebang from #!/bin/bash to #!/bin/sh (no bash-specific features used)

Closes #653

Test plan

• shellcheck github/cancel-workflows passes
• checkbashisms github/cancel-workflows passes
• Pre-commit hooks pass

[dnyw4l3n13]

PR marked as ready for review. CI checks are now running (previously skipped while in draft). No outstanding review comments or code issues to address.

[credfeto]

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[dnyw4l3n13]

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No review comments to address. PR is ready to merge.

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