infra(repo): add CODEOWNERS and branch protection ruleset for main

[marc0olo]

Summary

• Adds .github/CODEOWNERS assigning @dfinity/dx as the default code owner for all files
• Documents the accompanying branch ruleset applied directly to repo settings (ruleset ID: 15658343)

What changed

Branch ruleset — "Protect main" (active)

Applied via the GitHub Rulesets API. Rules enforced on main:

Rule	Setting
Direct pushes	Blocked — every change must go through a PR
Force pushes	Blocked
Branch deletion	Blocked
Required approvals	1, from a code owner (@dfinity/dx)
Stale review dismissal	Enabled — new commits invalidate existing approvals

Bypass actors: @dfinity/dx — team members can merge their own PRs without a separate review.

CODEOWNERS

All files (*) are owned by @dfinity/dx. Every PR will automatically request a review from the team, and the ruleset's "require code owner review" rule will be satisfied by any dx team member approving.

Automated workflows

The sync-motoko workflow opens PRs but never auto-merges, so it is unaffected. Those PRs will require a normal review from any dx member before merging.

Sync recommendation

hand-written