[PROPOSAL] Add SPDX License header and checker to YAML files

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What/Why

What are you proposing?

All source code should have an SPDX license header.

What users have asked for this feature?

Standard org-wide practice.

What problems are you trying to solve?

Org-wide practice says we need SPDX license headers everywhere.

What is the developer experience going to be?

CI fails if a source file (e.g. .ts) is missing a license header.

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License headers added to TypeScript files in #311, but we still need them in the YAMLs I think.