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.

索引モデルが issue の本文から書いたものです。