feat(cli): add omz generate plugin to scaffold custom plugins

Adds a rails-style generator that creates a custom plugin in
$ZSH_CUSTOM/plugins/<name> from commented templates: the plugin file,
a README in the usual shape and, for plugins that wrap a command-line
tool, a completion skeleton (or the cached-completion block when the
tool generates its own). Anything not passed as an option is asked
for interactively; non-interactive shells use defaults.

`omz generate` is a new top-level command so that other generators
(e.g. `omz generate theme`) can follow the same shape.

Templates live in templates/generators/plugin and are syntax-checked
by CI. lib/tests/generate-plugin.test.zsh covers the non-interactive
path, name validation and the prompt helper.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

templates/generators/plugin/README-cache.md-template

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+
+## Cache
+
+This plugin caches the completion script and automatically updates it when the
+plugin is loaded, which is usually when you start a new terminal emulator.
+
+The cache is stored at `$ZSH_CACHE_DIR/completions/_%command%`.

templates/generators/plugin/README-generic.md-template

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+# %name% plugin
+
+%description%
+
+To use it, add `%name%` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... %name%)
+```
+
+This plugin does not add any aliases.

templates/generators/plugin/README.md-template

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+# %name% plugin
+
+%description%
+
+To use it, add `%name%` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... %name%)
+```
+
+## Aliases
+
+| Alias        | Command            | Description           |
+| :----------- | ------------------ | :-------------------- |
+| `%command%s` | `%command% status` | Show %command% status |
+
+## Requirements
+
+This plugin requires [%command%](https://example.com) to be installed.
+%cache%

templates/generators/plugin/completion-cached.zsh-template

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+#
+# Completion
+#
+# %command% generates its own zsh completion, so we cache it instead of
+# maintaining a `_%command%` file by hand.
+#
+# On the first shell after installing this plugin the cache file doesn't exist
+# yet, so compinit hasn't bound it. Do that manually here.
+if [[ ! -f "$ZSH_CACHE_DIR/completions/_%command%" ]]; then
+  typeset -g -A _comps
+  autoload -Uz _%command%
+  _comps[%command%]=_%command%
+fi
+
+# Regenerate the cache in the background so it never blocks shell startup.
+# TMPPREFIX puts the temp file next to the destination, which keeps the move
+# atomic.
+zmodload -F zsh/files b:zf_mv
+() {
+  local TMPPREFIX="$ZSH_CACHE_DIR/completions/_%command%"
+  zf_mv -f -- =( %compgen% ) "$TMPPREFIX"
+} &|

templates/generators/plugin/completion-note.zsh-template

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+#
+# Completion
+#
+# Oh My Zsh adds this directory to $fpath, so a file named `_%command%` next to
+# this one is picked up automatically as the completion for %command%.
+# There is nothing to do here.
+#

templates/generators/plugin/completion.zsh-template

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+#compdef %command%
+#
+# Completion for %command%.
+#
+# This is a starting point. Delete what you don't need. Two references:
+#   https://zsh.sourceforge.io/Doc/Release/Completion-System.html
+#   https://github.com/zsh-users/zsh/blob/master/Etc/completion-style-guide
+
+local -a subcommands
+subcommands=(
+  'status:Show the current status'
+  'run:Run something'
+  'help:Show help for a command'
+)
+
+_arguments -C \
+  '(-h --help)'{-h,--help}'[show help]' \
+  '(-v --version)'{-v,--version}'[show the version]' \
+  '1: :->subcommand' \
+  '*:: :->args'
+
+case $state in
+  subcommand)
+    _describe -t commands '%command% subcommand' subcommands
+    ;;
+  args)
+    case $words[1] in
+      status) _arguments '--short[one-line output]' ;;
+      run)    _files ;;
+    esac
+    ;;
+esac

templates/generators/plugin/plugin-generic.zsh-template

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+# %name% plugin
+#
+# %description%
+#
+# This file is sourced by Oh My Zsh in every new shell, so keep it fast.
+# Avoid running external commands at the top level unless you really need to.
+#
+# Wrapping a command-line tool? Guard everything so people who don't have it
+# installed don't end up with broken aliases:
+#
+# if (( ! $+commands[sometool] )); then
+#   return
+# fi
+
+#
+# Aliases
+#
+# Document every alias in README.md. Short, obvious, and few beats many.
+#
+
+# alias %name%='echo "hello from %name%"'
+
+#
+# Functions
+#
+# Anything that needs arguments in the middle, or more than one command,
+# belongs here rather than in an alias.
+#
+
+# function %name%_hello() {
+#   echo "hello, ${1:-world}"
+# }
+
+#
+# Completion
+#
+# Oh My Zsh adds this directory to $fpath, so a file named `_sometool` next to
+# this one is picked up automatically as the completion for `sometool`.
+#
+
+#
+# Need the path to this plugin's own directory (for data files, a lib/ dir...)?
+# This is the standard way to get it, since $0 is not reliable on its own:
+# https://zdharma-continuum.github.io/Zsh-100-Commits-Club/Zsh-Plugin-Standard.html
+#
+# 0="${ZERO:-${${0:#$ZSH_ARGZERO}:-${(%):-%N}}}"
+# 0="${${(M)0:#/*}:-$PWD/$0}"
+# source "${0:A:h}/lib/helpers.zsh"

templates/generators/plugin/plugin.zsh-template

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+# %name% plugin
+#
+# %description%
+#
+# This file is sourced by Oh My Zsh in every new shell, so keep it fast.
+# Avoid running external commands at the top level unless you really need to.
+
+# Only define anything if the tool is actually installed. Without this guard,
+# people who don't have %command% get aliases that fail with "command not found".
+if (( ! $+commands[%command%] )); then
+  return
+fi
+
+#
+# Aliases
+#
+# Document every alias in README.md. Short, obvious, and few beats many.
+# The one below is just an example: replace it with something %command% can do.
+#
+
+alias %command%s='%command% status'
+
+#
+# Functions
+#
+# Anything that needs arguments in the middle, or more than one command,
+# belongs here rather than in an alias.
+#
+
+# function %name%_current() {
+#   %command% status --short "$@"
+# }
+
+%completion%
+
+#
+# Need the path to this plugin's own directory (for data files, a lib/ dir...)?
+# This is the standard way to get it, since $0 is not reliable on its own:
+# https://zdharma-continuum.github.io/Zsh-100-Commits-Club/Zsh-Plugin-Standard.html
+#
+# 0="${ZERO:-${${0:#$ZSH_ARGZERO}:-${(%):-%N}}}"
+# 0="${${(M)0:#/*}:-$PWD/$0}"
+# source "${0:A:h}/lib/helpers.zsh"