fix: make edit add label generate the labels field instead of deprecated commonLabels

[jojinkb]

What this PR does / why we need it:

kustomize edit add label environment:dev wrote the deprecated commonLabels field, so kustomize build immediately printed a deprecation warning about configuration the CLI itself had just generated (#5726, triage/accepted).

With this change, edit add label (no flags) appends to the labels field with includeSelectors: true, which is semantically identical to commonLabels — kustomize build output is byte-identical between the two forms, but no deprecation warning is triggered:

# kustomization.yaml after `kustomize edit add label environment:dev`
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
labels:
- includeSelectors: true
  pairs:
    environment: dev

Design decisions (flagging explicitly for review):

• Backward compatibility: if the kustomization file already uses commonLabels, the command keeps updating commonLabels in place, so a single file never ends up mixing both fields. kustomize edit fix remains the migration path. This means existing files/scripts see no change; only freshly-generated configuration switches to the non-deprecated field, per the maintainer guidance on the issue ("update the existing command to do so"). The alternative — a new opt-in flag — would keep emitting deprecated config by default, which is what the issue asks to stop.
• edit remove label now removes keys from both commonLabels and every labels entry (dropping entries left without pairs), so labels created by the new default remain removable and the command family stays round-trippable. Previously, labels in the labels field could not be removed via the CLI at all.
• --without-selector / --include-templates behavior is unchanged. edit set label is intentionally untouched, since feat: add support to set labels without selector #6039 covers it (only overlap is an identical one-line help-string change).

Which issue(s) this PR fixes:

Fixes #5726

Testing:

• New TestAddLabelGeneratesLabelsField reproduces the issue's exact scenario end to end; new table-driven TestAddLabelDefaultsToLabelsField covers the new default, appending to an existing includeSelectors entry, not mixing with --without-selector entries, and the commonLabels back-compat path.
• New remove-side tests: removal from the labels field (with empty entries dropped), removal spanning both fields, file-level removal leaving no empty labels: key, and the updated "neither field defined" error.
• All 6 new/updated tests fail without the fix. go test ./commands/... (kustomize module) passes; golangci-lint run (repo-pinned v1.64.8) is clean on the changed packages.
• Manually verified with a built binary: fresh file gets the issue's expected output; build emits no warning; commonLabels vs new labels form produce identical rendered resources; legacy commonLabels files keep their format on add.

/kind cleanup
/kind deprecation

This change was developed with AI assistance (Claude Code); I have reviewed and tested it.

[kubernetes-prow]

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This pull-request has been approved by: jojinkb
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[kubernetes-prow]

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