feat: add --zone|-z flag to plan and apply for single-zone targeting

[cl8dep]

Summary

Closes #58

Adds -z|--zone <ZONE> to both plan and apply (via PlanSettings, which ApplySettings inherits). When provided, only the specified zone is processed and only its source/target providers run preflight checks — all other zones and providers are skipped.

Usage

# Plan only one zone
dns-sync plan -c config.yaml --zone example.com.

# Apply only one zone without confirmation
dns-sync apply -c config.yaml --zone example.com. --yes

# Zone not in config → clear error, exit 1
dns-sync plan -c config.yaml --zone unknown.com.
# ✗ Zone 'unknown.com.' not found in config.

Test plan

• plan --zone example.com. → only that zone is planned, preflight skips other providers
• apply --zone example.com. --yes → only that zone is applied
• plan --zone nonexistent.com. → exit 1 with error message
• plan without --zone → all zones processed as before (no regression)
• dotnet test passes (188 tests)