Keep wt numbering stable when agent worktrees exist

[mattmenefee]

Summary

• git worktree list sorts linked worktrees by directory basename, so the throwaway checkouts Claude Code creates for its subagents under .claude/worktrees/agent-<id> sorted in among the permanent worktrees rather than after them. A running agent renumbered the list mid-session, and wt 2 stopped resolving to the same checkout it meant a moment earlier
• Adds a wtlist helper that holds the temporary entries back and prints them after everything else. Agent worktrees stay visible and reachable by number without displacing the permanent ones ahead of them
• wt resolves paths from git worktree list --porcelain -z, read into an array. NUL-delimited records are the only format that keeps a path containing a space or a newline intact — plain --porcelain leaves a newline-bearing path split across two lines, which would invent an entry and pull the numbering out of step. wtl keeps reordering git's human-readable listing so it retains the commit and branch columns, matching against the whole line, which is equivalent to matching the path because git forbids a refname component beginning with a dot
• Indexing an array also retires sed -n "${1}p", which was a sed program rather than a subscript: wt '1w /some/file' truncated an arbitrary file, wt '1r /some/file' read one back, and a bare wt printed every line into cd. A numeric guard now turns away anything that is not a plain number
• Both refusals report on stderr and return non-zero. A missing worktree used to be announced on stdout while wt still exited successfully, so wt 9 && bin/dev went on to run the command from wherever the shell already stood
• The loop variable in _wt_paths is named dir rather than the obvious path because zsh ties path to PATH — a local by that name empties it and git stops resolving inside the function. There is a comment on the declaration so a future tidy-up does not reintroduce it
• Split out of Isolate review subagents in throwaway git worktrees #91, which needs this fix but is otherwise about isolating review subagents. Isolate review subagents in throwaway git worktrees #91 will be rebased to drop its copy of this change

Requires source ~/.zshrc to pick up the new functions; shells already running keep the old wt.

Test plan

• zsh -n home/.zshrc parses
• Ordering verified in a scratch repo holding a permanent worktree, one under a path containing a space, one under a path containing a newline, and an agent worktree nested beneath My Code/.claude/worktrees/ — wtl lists the agent entry last, and the earlier index($1, ...) form left it interleaved because $1 stops at the first space
• wt N lands in the directory wtl numbers N for every entry, including the space-containing and newline-containing paths that the previous awk '{print $1}' truncated
• Confirmed against git 2.55.0 that git worktree list --porcelain without -z splits a newline-bearing path across two lines while -z keeps the record whole, and that the human-readable listing C-quotes it onto one line — the reason the two consumers read different formats
• wt abc, wt -1, wt '', wt '1p;2', and wt '1w /tmp/CLOBBER_ME' are all refused with Usage: wt <number> and exit 1, and the w attempt created no file
• wt 0 and wt 99 report not found on stderr and exit 1
• wt 2 && … runs the following command and wt 99 && … does not, with wt 99 || … firing as expected
• wtl surfaces fatal: not a git repository again outside a repository, where the first revision of this branch printed nothing at all
• Numbering and path resolution checked against the real dotfiles worktrees
• No line exceeds 100 characters