CI: Limit pipeline to pre-commit on [REDUCED CI] MR title and/or Reduced CI label

Limit the CI pipeline to only the pre-commit and doxygen-documentation jobs when the merge request opts in, either via a case-sensitive [REDUCED CI] marker in the title or a Reduced CI label.

What

• New reusable .reduced-ci rule: when: never for a job when $CI_MERGE_REQUEST_IID && ($CI_MERGE_REQUEST_TITLE =~ /\[REDUCED CI\]/ || $CI_MERGE_REQUEST_LABELS =~ /Reduced CI/).
  • The [REDUCED CI] title marker is case-sensitive, so it is clear something magic is going on.
  • The Reduced CI label is cased to match the existing Allow API ABI Break label.
• Jobs that should be skippable reference *reduced-ci explicitly in their rules, alongside *manual-on-draft where they need both — the skip logic is not baked into the .manual-on-draft anchor.
• pre-commit and doxygen-documentation carry no skip rule, so they still run (the documentation stays browsable via its environment even on reduced-CI MRs).
• Tag / default-branch-only jobs are unaffected (rule guarded by $CI_MERGE_REQUEST_IID).

Relies on GitLab flattening nested rule arrays, already used by - *manual-on-draft.

Closes #1687 (closed)

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