chore(ci): align cache pin documentation and retired path filter

[jmagar]

Problem and resolution

Preserve a previously committed local CI correction while consolidating work onto main. The setup action comment still names cache version 0.15.1 although its configured pin is 0.28.1, and the main push trigger includes the retired OpenWiki subtree.

Update the comment to the configured version and exclude openwiki/** from push-triggered CI. Pull request triggers remain unchanged.

Verification and risk

The diff contains two line additions and one line removal. The cache executable version is unchanged. Both files live in .github, which the Docker build context excludes, so this correction does not change the deployed application bytes.

Focused independent review found no issues. Local pre-push checks passed: actionlint, 32 changed-path tests, 81 workflow-shape tests, and structural checks. Exact-head hosted CI passed, including all three platform smokes and CodeQL configurations; workspace tests passed 6,587 with 9 skipped. Merged as f2831126da7d0228874a4a4f2475f3a7942d6018. The application Docker context matches tested source de412c30a.

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