git worktree list parsing gets duplicate-path and NUL-separated output handling

What

Parsing of git worktree list --porcelain output was made more robust in two ways:

• A new validation check looks for duplicate worktree paths in the parsed listing, normalizing paths to NFC on macOS first, and is run as a standalone check before the usual branch-matching scan; if it finds duplicates or an inconsistent listing, the scan is skipped and treated as unreliable.
• Worktree listing now probes whether the installed git supports NUL-separated output and, when it does, passes -z to git worktree list --porcelain and splits entries on the null character instead of newlines, avoiding ambiguity when a worktree path itself contains a newline.

Why

Both changes prevent Claude Code from misreading the worktree list when paths are duplicated, unicode-normalized differently, or contain unusual characters like newlines, which could otherwise cause it to match the wrong worktree.