Investigate commit field serialization in pipeline trigger requests

[Copilot]

The review comment raised concerns that ReferenceTarget.Commit serializes with empty fields when --commit is not provided, potentially creating invalid API payloads.

Investigation

Verified current behavior: when triggering without --commit, the request includes:

"commit": {
  "type": "commit",
  "hash": ""
}

This occurs because:

• Commit field is a non-pointer struct without omitempty
• CommitReference.MarshalJSON() always sets type: "commit" regardless of hash value

Clarification Requested

@gildas stated "the commit is never empty" - asked for clarification on whether:

1. Bitbucket API accepts empty hash and defaults to branch HEAD
2. CLI should fetch branch HEAD commit before sending
3. Field should be omitted entirely with omitempty

No code changes made pending clarification of intended behavior.

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