feat: add configurable MergeStrategy to CommitConfiguration and ChangeTransferPolicy

[Copilot]

Promotions were always using a hardcoded merge strategy, leaving all individual and merge commits visible in the target branch history instead of allowing them to be squashed.

Changes

• New type PullRequestMergeStrategy (merge | squash) added to pullrequest_types.go; defaults to merge for full backward compatibility
• CommitConfiguration gains a MergeStrategy field with kubebuilder enum validation and default
• ChangeTransferPolicySpec gains a Commit CommitConfiguration field so the strategy is configurable at the CTP level
• CTP controller propagates Commit.MergeStrategy into the PullRequest spec on both creation and merge transition
• Generated files updated via make generate && make manifests (deepcopy, applyconfiguration builders, CRD YAML)

Example

apiVersion: promoter.argoproj.io/v1alpha1
kind: ChangeTransferPolicy
spec:
  commit:
    mergeStrategy: squash  # or "merge" (default)

SCM provider-level squash implementation (GitHub, GitLab, Forgejo, etc.) is not included here — the field is stored and propagated correctly; provider support is a follow-up.

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