Define operator conventions for domain operators

[DmyMi]

Description

Document the contract all three domain operators follow so implementations stay consistent without sharing a single codebase. No operator binary in this issue—only docs, Makefile stubs, and CI hooks teams will use.

Scope

• Add docs/k8s/operators-overview.md (and link from docs/k8s/operator.md):
  • Each team owns operators/<domain>-operator/ + cmd/<domain>-operator/main.go (or equivalent)
  • Shared spec shape: replicas (int32), enabled (bool); optional deploymentName defaulting to team's Deployment
  • Shared status: conditions[] with type Ready
  • Reconciler behavior: patch target Deployment replicas; enabled: false → 0; surface errors in status
  • Naming: API group per domain (guest.sharebite.dev, business.sharebite.dev, admin.sharebite.dev)
• Makefile targets: run-guest-operator, run-business-operator, run-admin-operator, test-operators (runs all three when present)
• CI: extend workflow to run go test ./operators/... (passes with no packages until team PRs land)
• Optional: deploy/k8s/operators/README.md listing three subfolders

Acceptance criteria

• Overview doc is enough for a team to start their operator issue without reading other teams' code.
• Makefile targets exist (can noop or skip missing dirs until operators merge).
• No merge order required between team operator PRs.

Blocks: guest-operator, business-operator, and admin-operator implementation issues.

Part of #207