Problem

Sibling/Internal layout path computation logic is copy-pasted 3 times:

• manager.rs:65-81 (create)
• manager.rs:254-269 (adopt)
• commands.rs:1561-1575 (undo)

Proposed Solution

Extract into a single WorktreeManager::compute_path(&self, ticket: &str) -> PathBuf method.

Category: refactor | Priority: medium