Consolidate workflow pushes for unborn repositories

[carlospedreira]

Problem

PR #662 consolidates remote pushes for workflow runs that have an integration branch, but supported repositories with no commits do not create one. In that path, $integrationBranch remains null for the whole run, so Complete-TaskWorktree is invoked without -SkipRemotePush and each completed task pushes the newly established base branch. A multi-task run in an unborn repository therefore still triggers one remote push/CI run per task.

The same workflow merge logging also falls back to the literal main when no integration branch is present, even though the actual recorded base can be master or a configured git.base_branch.

Follow-up from #662.

Acceptance criteria

• A multi-task WorkflowRun started in an unborn repository performs at most one remote push for the run, after task merges have been consolidated.
• Standalone/pending-task completion continues to push its actual recorded base branch immediately.
• Existing committed-repository WorkflowRuns continue to suppress per-task integration-branch pushes and push the integration branch once at run completion.
• Failure/no-remote behavior preserves local commits and reports an actionable push status without losing work.
• Merge status/activity messages use the actual recorded or resolved merge target rather than a literal main, covering master and configured git.base_branch.
• Regression coverage uses a local bare remote and verifies push/ref-update counts for an unborn multi-task run, a normal committed multi-task run, and standalone completion.