Restore main as the bleeding-edge branch

[lorisleiva]

This PR follows the branch rename back to main, reverting the mainless experiment in favour of a permanent main branch: .changeset/config.json points back at main and the workflow triggers accept main alongside the numeric major-branch pattern. It also adds the repository's missing CONTRIBUTING.md, documenting the day-to-day flow and linking the ecosystem-wide release process.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 1f18870

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