fix: pass worktree branch name from New Task dialog

[sergioestebance]

Summary

• The "Branch name" field in the New Task dialog was displayed in the command preview but never passed through to the actual createManualTaskAndLaunch function
• All worktrees created via New Task got random names instead of the user-specified branch name
• Threads worktreeBranch through the onCreateAndLaunch callback → createManualTaskAndLaunch → executeLaunch, matching how LaunchConfirmationDialog already handles it correctly

Test plan

• Open New Task dialog, enable "Create worktree", enter a branch name (e.g. fix/my-branch)
• Click "Create & Start" and verify the launched claude command includes --worktree fix/my-branch
• Repeat with an empty branch name field and verify a random worktree name is generated
• Verify launching via LaunchConfirmationDialog still works correctly (no regression)