Preserve the active branch when delegating to the cloud agent

[Copilot]

Delegation to the cloud agent could silently switch the PR base from the user's current branch back to the repository default branch. This broke continuation from branch-specific work unless the branch had already been explicitly reselected through repository resolution.

• Branch selection

  • Preserve an existing base_ref when delegating within the current repository.
  • Re-resolve the base branch only when:
    • no base_ref is available, or
    • the user selected a different repository for delegation.

• Resolution guardrails

  • Extract the base-branch decision into a small helper to make the behavior explicit.
  • Ignore malformed selected-repository values instead of treating them as a repository switch.
  • Handle cases where current repository metadata is unavailable by falling back to branch resolution.

• Focused coverage

  • Add unit coverage for:
    • current repo + existing branch
    • default repository selection
    • missing base_ref
    • alternate selected repository
    • malformed repository identifiers
    • missing current repository metadata

if (!baseRefToUse || shouldResolveDelegationBaseRef(baseRefToUse, { owner: repoOwner, name: repoName }, selectedRepository)) {
	const { default_branch } = await this._githubRepositoryService.getRepositoryInfo(repoOwner, repoName);
	baseRefToUse = default_branch;
}

This keeps cloud delegation aligned with the branch the user is actually working on, while preserving the previous fallback behavior for cross-repo delegation and missing branch state.