fix(workspace): improve delete responsiveness

[reckerp]

Summary:

• load workspace status asynchronously so the picker renders immediately
• cap concurrent status scans and cancel pending scans when the picker exits
• reuse loaded dirty status to avoid an unnecessary failed remove pass

Verification:

• cargo test
• cargo clippy --all-targets -- -D warnings
• git diff --check
• rustfmt --edition 2024 --check src/git/worktree.rs src/ui/workspace_picker.rs src/commands/workspace.rs