enhancement(worktree): add #[non_exhaustive] to WorktreeError

[bug-ops]

Description

WorktreeError in crates/zeph-worktree/src/error.rs:12 is a public enum without
#[non_exhaustive]. This violates the workspace convention established by PR #4658
(all extensible public enums must carry #[non_exhaustive] to preserve forward
compatibility for downstream consumers and plugin code).

Fix

Add #[non_exhaustive] to the WorktreeError enum:

#[non_exhaustive]
#[derive(Debug, thiserror::Error)]
pub enum WorktreeError { ... }

Environment

• File: crates/zeph-worktree/src/error.rs:12