feat: wt delete takes more than one worktree

[lorenzolfm]

wt delete eng-1234 eng-5678 removes both. Two reviews land in the same afternoon, and the tool asked for wt delete twice.

The rules

• Every name is read first. A name that is not a worktree stops the command with the list of names, and nothing is removed. A list is typed in one line and read back in one glance, so a typo in the second name is a list you did not mean to run.
• A repetition is one worktree. A name and the path of the same worktree name one worktree; the second git worktree remove of it would fail with a reason that says nothing.
• A refusal does not stop the list. Git prints its own reason, wt collects the name, and the command fails at the end naming each one, so the exit code still says the list did not run in full.
• The count prints for a list only. One name gives one line, and that line is the whole report. Output for a single worktree is unchanged.

Notes

resolve takes the worktrees and the names and returns the choice, so order, repetition and the unknown name are unit tests that need no repository. The fish completions need no change: they already offer a worktree name after delete and d, at each position.

Verified

In a scratch repository: an unknown name removes nothing (exit 1); two clean worktrees go together; a dirty worktree is kept while its clean neighbour is removed, with both reported (exit 1); wt delete with no argument is a clap usage error. cargo clippy --all-targets -- -D warnings is clean, 38 tests pass.