fix: quote colon-containing description in publish-single-page-docs action manifest

[oto-macenauer-absa]

Summary

The github-token input description in actions/publish-single-page-docs/action.yml was written as an unquoted plain scalar containing contents: write. In YAML, a : inside a plain scalar starts a nested mapping, so GitHub's action-manifest parser rejected the file with "Mapping values are not allowed in this context" at Set up job time — before any step ran. That broke every workflow consuming the action.

Failing run: https://github.com/AbsaOSS/organizational-workflows/actions/runs/30910992670

Fix

Moved the description into a >- folded block, matching the style already used by the other input descriptions in the same file:

  github-token:
    description: >-
      Token used to upload the release asset. Needs `contents: write`.

Nothing else in the manifest changed.

Validation

• Parsed actions/**/*.yml and .github/workflows/*.yml with PyYAML 6.0.3 — all five files parse cleanly.
• Confirmed the pre-fix line reproduces the failure: PyYAML reports mapping values are not allowed here on the original scalar.
• Scanned the same file set for other unquoted plain scalars containing : (the identical bug pattern); the detector was sanity-checked against the known-bad line and found no further instances.

Also: CI now validates action manifests

Nothing in CI would have caught this. The repository's own workflows are valid YAML, but action.yml is a consumer-facing manifest that is only parsed when a downstream repository runs the action — which is why the break surfaced in organizational-workflows rather than here.

This PR therefore also adds a Validate action manifests step to the existing publish-single-page-docs CI job, which YAML-parses every actions/**/*.yml and actions/**/*.yaml file and fails the build on a parse error, printing each file it checked so a passing run is auditable.

It parses with the yaml@2.7.1 package the action already pins and the job already installs, so there is no new dependency, no new config file and no extra runner — and it is the same parser the action uses at runtime. node_modules is excluded from the walk, and an empty match set is treated as a failure so the check cannot silently pass if the layout changes.

Verified by extracting the run script straight out of ci.yml and executing it verbatim:

• On this branch it passes (exit 0): ok actions/publish-single-page-docs/action.yml
• Re-introducing the broken line fails it (exit 1) with Nested mappings are not allowed in compact mappings at line 26, column 18 — confirming the gate actually catches the bug this PR fixes, rather than passing vacuously.

Fixes #39

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https://claude.ai/code/session_01CRZk4ZQsApxLANJtzQpVXb