Function merge_single_spec 

pub fn merge_single_spec(
    spec_id: &str,
    spec_branch: &str,
    main_branch: &str,
    should_delete_branch: bool,
    dry_run: bool,
) -> Result<MergeResult>

Merge a single spec’s branch into the main branch.

This function:

1. Saves the current branch
2. Checks if main branch exists
3. Checks out main branch
4. Merges spec branch with fast-forward only
5. Optionally deletes spec branch if requested
6. Returns to original branch

In dry-run mode, no actual git commands are executed.