Configure CODEOWNERS and/or required reviewers for intercom-manager and intercom-frontend

[birme]

Split from #297 by daily-backlog-pr triage (2026-09-15).

Define who should review PRs in each repo. Consider a GitHub CODEOWNERS file to auto-request reviewers, or adjust branch protection rules to assign reviewers.

Acceptance: CODEOWNERS file exists and is enforced, or branch protection rules explicitly assign reviewers.

Context: #297 found essentially every open PR sits with zero human reviews (reviewDecision: REVIEW_REQUIRED), oldest since Nov 2025.