Split merge-main-into-dev: docs and CI changes

[ilml]

Summary

• Extract the documentation, workflow, and review-tooling updates from Merge main into dev #3865 into a separate PR.
• Includes GitHub/GitLab CI changes, docs and README updates, codecov config, and the internal CI trigger tooling.
• Excludes runtime code and test updates.

Test plan

• Not run locally; this branch was split mechanically from m2d
• Diff bucket validated against the original m2d branch

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