[P1, hard] Adopt `codeowners-plus` GitHub Action for better CO reviews - smart dismissals, enforcement of complete reviews

## Problem

PR authors can request codeowner (CO) reviews without having any "complete review". If multiple codeowners approve only their related changes, the PR can get merged without a proper complete review of the entire changeset.

### Example Scenarios

1. A PR touches files owned by Team A and Team B
2. Team A's codeowner approves only the files they own
3. Team B's codeowner approves only the files they own
4. PR merges without anyone reviewing the complete PR holistically

### Edge Case

There's a legitimate scenario where a team reviews a complete PR that is also CO-owned by that same team. Any enforcement mechanism needs to account for this.

## Proposed Solution

Adopt [codeowners-plus](https://github.com/multimediallc/codeowners-plus) GitHub Action, which provides several features that address this gap:

| Feature | Description | How It Helps |
|---------|-------------|---------------|
| **AND codeowner rules** | Require approval from multiple specified owners | Ensures comprehensive review coverage |
| **`unskippable_reviewers`** | Specify reviewers that cannot be skipped via `max_reviews` setting | Guarantees certain reviewers always review |
| **`require_both_branch_reviewers`** | Requires approval from codeowners defined in BOTH base branch AND PR branch | Prevents bypassing via branch-specific rules |
| **Smart dismissal** | Only dismiss approval when owned files change | Reduces re-review churn for unrelated changes |

## Context

- GitHub has an upstream feature request for native AND rules but no timeline: [community/discussions/166110](https://github.com/orgs/community/discussions/166110)
- Previous approach ([code-captains](https://github.com/upshift-dev/code-captains)) appears abandoned
- As the codebase and teams grow, this becomes increasingly important

## Tasks

- [ ] Evaluate `codeowners-plus` action in a test repository
- [ ] Define AND rules for critical paths (e.g., security-sensitive code)
- [ ] Determine `unskippable_reviewers` list for complete review enforcement
- [ ] Draft updated CODEOWNERS configuration
- [ ] Test edge case: team reviewing complete PR that they also CO-own
- [ ] Roll out to metamask-extension repository
- [ ] Document new review workflow for contributors

## References

- [codeowners-plus](https://github.com/multimediallc/codeowners-plus) - 33 stars, actively maintained (Go)
- [GitHub AND rules discussion](https://github.com/orgs/community/discussions/166110)
- [code-captains](https://github.com/upshift-dev/code-captains) - appears abandoned (TypeScript)

## Open Questions

1. How do we handle the case where a team legitimately reviews a complete PR they also CO-own?
2. Should we phase this in gradually (starting with critical paths) or adopt comprehensively?
3. What's the impact on PR velocity and reviewer load?