Add shell auto-completion for the wt command

[ukanga]

What to build

Add static shell auto-completion for wt covering bash, zsh, fish, and elvish. Completion is generated from the existing clap command tree via clap_complete, exposed through a hidden wt completions <shell> subcommand that prints the script to stdout. install.sh is updated to install the appropriate script into the user's shell completion directory during installation.

End-to-end behaviour after this lands:

• wt <TAB> completes to the available subcommands (new, use, ls, rm, which, session).
• wt session <TAB> completes to ls, add, rm, watch.
• wt session --mode <TAB> completes to panes, windows.
• Flags (-b, --print-path, --panes, --watch, -d, --dir) complete after their parent subcommand.
• Long options like wt new --pri<TAB> complete to --print-path.

Because the completion script is generated from the live clap::Command tree, subcommands and flags stay in sync automatically as the CLI evolves — no hand-maintained completion files.

Out of scope (file as follow-ups)

• Dynamic completion of worktree names for wt use, wt rm, wt session rm (requires shell wrapper functions or clap_complete's unstable dynamic feature).
• PowerShell support, gated on shipping Windows release binaries.

Implementation notes

• Add clap_complete = "4" as a dependency.
• Add a hidden wt completions <shell> subcommand (#[command(hide = true)]) where <shell> is clap_complete::Shell. The handler calls clap_complete::generate against the same root Cli command and writes to stdout.
• Extend install.sh to install the completion script for the detected shell:
  • bash → ${BASH_COMPLETION_USER_DIR:-$XDG_DATA_HOME/bash-completion}/completions/wt, falling back to ~/.local/share/bash-completion/completions/wt.
  • zsh → a directory on fpath (e.g. ~/.zsh/completions/_wt); if the dir is not on fpath, append a single guarded fpath=(~/.zsh/completions $fpath) line to ~/.zshrc ahead of any compinit (mirror the existing alias-install idempotency pattern with grep -q).
  • fish → ~/.config/fish/completions/wt.fish.
  • elvish → document the snippet to add to ~/.config/elvish/rc.elv (elvish loads completions via rc.elv, not from a magic dir).
• Print a one-line confirmation per shell, mirroring the existing alias-install messaging style.
• Document wt completions <shell> in the README so users with non-standard setups can pipe it to a custom location.

Acceptance criteria

• clap_complete is added as a dependency.
• wt completions <shell> (hidden subcommand) emits a valid completion script for bash, zsh, fish, and elvish to stdout and exits 0.
• The subcommand does not appear in wt --help.
• install.sh installs the completion file for the detected shell (bash / zsh / fish) into the correct location and is idempotent (re-running does not append duplicate fpath lines or duplicate completion files).
• For zsh users whose fpath does not already contain the install directory, install.sh adds the necessary fpath line to ~/.zshrc exactly once, ahead of any existing compinit call.
• README.md documents (a) automatic install for bash/zsh/fish, (b) the elvish snippet users must add by hand, and (c) the manual wt completions <shell> > <path> escape hatch.
• An integration test invokes the completions subcommand for each supported shell and asserts the output is non-empty and contains the literal subcommand names (new, use, ls, rm, which, session).
• PR description records manual verification: in fresh bash, zsh, and fish shells, wt <TAB> lists subcommands and wt session --mode <TAB> lists panes and windows.

Blocked by

None — can start immediately.