Trigger Workflow preselects stale/unrelated branch instead of active workspace branch

### Extension
GitHub Actions for VS Code

### Summary
When triggering a workflow from VS Code, the branch/ref dropdown preselects an unrelated branch (often from a linked worktree or prior context) instead of the branch currently checked out in the active workspace folder.

### Expected behavior
Default selected branch/ref should be the branch currently checked out in the active workspace repository.

### Actual behavior
The dialog defaults to a different branch than the active workspace branch.

### Repro (generic)
1. Create a repository with multiple branches.
2. Create at least one additional Git linked worktree with a different branch checked out.
3. In VS Code, open the original workspace folder on branch `feature/current`.
4. Use the extension command/UI to trigger a workflow manually.
5. Observe the preselected branch/ref in the trigger dialog.

### Observed result
The preselected branch/ref is not `feature/current`; it appears to come from stale/cached/worktree context.

### Notes
- Manually switching the branch in the dialog works, but the default is misleading and easy to miss.
- This can cause accidental workflow runs on the wrong branch.

### Suggested fix
Prefer branch/ref resolution from the active workspace repository HEAD (and only fall back to cached/worktree-derived refs if workspace HEAD cannot be determined).

Start at the extension command or UI that opens the workflow trigger dialog and trace how its default branch/ref is resolved. Reproduce with an active workspace branch and a linked worktree, then verify the dialog prefers the active workspace repository HEAD and falls back only when that HEAD cannot be determined.

Written by the indexing model from the issue text.