Add Dependabot and CODEOWNERS, SHA-pin validate's actions, pin docs-check to v1.2.3

[cpitzi]

Part of lentago/shared-workflows#64

Origin: the fleet shared-workflows pinning audit (ADR-0005). The repository was created on 2026-10-04 without the fleet's standard Dependabot and CODEOWNERS files, with tag-pinned actions in validate.yml, and on docs-check@v1.2.2.

What changed

• .github/dependabot.yml (new): weekly github-actions updates, copied from drosera's file so the grouping matches the fleet: actions-routine (minor and patch) and actions-major, with open-pull-requests-limit: 5. No other ecosystem applies; the only dependencies are pip install pyyaml jsonschema inside CI.
• validate.yml SHA pins (fleet convention, .github#113):
  • actions/checkout@v4 → 11d5960a326750d5838078e36cf38b85af677262 # v4.4.0, the commit the v4 tag resolves to (lightweight tag; v4.4.0 points at the same commit). No fleet repository pins a v4 checkout; the fleet's common checkout pin is v7.0.1. Pinning the existing major keeps this change behaviour-neutral; the move to v7 is left to the actions-major Dependabot group added here.
  • actions/setup-python@v5 → a26af69be951a213d495a4c3e4e4022e16d87065 # v5.6.0, the commit v5 resolves to and the same pin mitchella already uses.
• docs-check.yml: lentago/shared-workflows/.github/workflows/docs-check.yml@v1.2.2 → @v1.2.3 (commit 43b4fe4); the stale # v1.2.1 comment is corrected.
• .github/CODEOWNERS (new): * @cpitzi, the same as drosera and kalmia.

Verification

• Tag SHAs resolved with gh api repos/actions/{checkout,setup-python}/git/ref/tags/<tag> (both lightweight tags pointing directly at commits) and cross-checked against matching-refs/tags/v4. / v5..
• The validate and docs-check checks on this PR exercise both changed workflows.

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📥 Commits

Reviewing files that changed from the base of the PR and between 7ac6ea9 and 3c41d0f.

📒 Files selected for processing (4)

• .github/CODEOWNERS
• .github/dependabot.yml
• .github/workflows/docs-check.yml
• .github/workflows/validate.yml

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