feat: install bash and zsh completions for gt

[sumanthratna]

Install bash and zsh completions for gt

brew install withgraphite/tap/graphite currently installs no shell completions, even though the CLI ships a gt completion command. This PR generates and installs them at install time using Homebrew's standard generate_completions_from_executable helper.

Staleness isn't a concern with this approach: the script yargs emits is a thin shim that invokes gt --get-yargs-completions <words> live on every Tab press, so the installed file never goes out of date as gt's commands evolve — it only needs to be installed once.

Supersedes #3

Credit to @jalaziz for the original attempt in #3. Investigating why that diff "didn't work as expected", I found two root causes, both fixed here:

1. feat: Generate shell completions on install #3 requested :fish, which yargs cannot emit. gt is built on yargs, whose completion command only generates bash or zsh scripts, selected via the SHELL environment variable (which generate_completions_from_executable sets per shell). Under SHELL=fish, yargs falls back to emitting a bash script, which Homebrew then installed into fish's completions directory — a broken fish "completion" that fails the moment it's exercised. This PR explicitly passes shells: [:bash, :zsh] (the helper's default list includes fish, so it must be overridden).

2. The yargs zsh script only registers the completer. It ends with compdef _gt_yargs_completions gt and never invokes the function (Generate zsh autoload completion function file yargs/yargs#2402; the #compdef gt header came in Zsh completion is now installable system-wide yargs/yargs#1856). When zsh autoloads _gt from fpath on the first Tab of a session, the file runs, registers the completer, and returns — completing nothing on that first press. This PR appends a _gt_yargs_completions "$@" invocation to the installed _gt so the first Tab works.

What the diff does

• Adds an install_shell_completions helper, called from each of the three def install blocks, in the template (formula-templates/graphite.rb) and both rendered formulas (Formula/graphite.rb, Formula/graphite-beta.rb) so the release pipeline won't clobber the change on the next version bump.
• shell_parameter_format: :none — yargs keys on the SHELL env var the helper sets; it would reject gt completion bash.
• chmod 0555, bin/"gt" before generating — the downloaded release artifact carries no exec bit, and Homebrew normalizes permissions only after install returns, so without it the helper fails with brew: exec failed (EACCES) (reproduced during testing).
• Appends the zsh invocation line described above.

Per the discussion in #4, the diff is intentionally minimal — no unrelated audit cleanups, no changes to version/test.

Testing (macOS arm64, gt 1.8.6)

Installed the patched formula via a local tap (current Homebrew rejects brew install --formula <path>); install succeeds and Homebrew reports the completions:

==> Reinstalling sumanthratna/completions-test/graphite
🍺  /opt/homebrew/Cellar/graphite/1.8.6: 5 files, 86.8MB, built in 1 second
==> Caveats
zsh completions have been installed to:
  /opt/homebrew/share/zsh/site-functions

Generated zsh script is the registration shim plus the appended invocation:

$ head -1 /opt/homebrew/share/zsh/site-functions/_gt
#compdef gt
$ tail -1 /opt/homebrew/share/zsh/site-functions/_gt
_gt_yargs_completions "$@"

Bash variant landed in bash's dir (and is actually bash, not zsh):

$ grep "complete -o" /opt/homebrew/etc/bash_completion.d/gt
complete -o bashdefault -o default -F _gt_yargs_completions gt

No file was written into share/fish/vendor_completions.d/ (fish correctly excluded).

A clean zsh registers the completer:

$ zsh -fc 'fpath=("$(brew --prefix)/share/zsh/site-functions" $fpath); autoload -Uz compinit && compinit -D; print -r -- ${_comps[gt]}'
_gt

The live query the shim performs on each Tab returns the subcommand list:

$ gt --get-yargs-completions gt ""
completion:Set up `bash` or `zsh` tab completion.
abort:Aborts the current Graphite command halted by a rebase conflict.
absorb:Amend staged changes to the relevant commits in the current stack.
add:git add passthrough
...

And interactively, gt <Tab> completes on the first press of a fresh shell session (the failure mode from root cause 2).

brew style reports no offenses for the added lines (the one pre-existing ComponentsOrder warning is untouched, keeping this PR minimal).

Question for maintainers

This repo's src/index.ts renders the template into Formula/graphite-beta.rb only, and stable promotion appears to happen via a separate mechanism. If that pipeline renders Formula/graphite.rb from a template copy living outside this repo, that copy needs the same change or the next stable release will revert it — happy to adjust whatever is needed.