feat: add fnox plugin w/ optional mise integration

plugins/fnox/README.md

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+# fnox
+
+Adds integration and shell completions for [fnox](https://github.com/jdx/fnox), a tool for provider-agnostic secret injection.
+Supports either standalone activation or `mise` integration if installed.
+
+## Installation
+
+1. *Optional:* [Download & install mise](https://github.com/jdx/mise#installation) by running the following:
+
+```bash
+curl https://mise.jdx.dev/install.sh | sh
+```
+
+2. [Enable fnox](https://fnox.jdx.dev/guide/quick-start.html) by adding it to your `plugins` definition in `~/.zshrc`.
+
+```bash
+# standalone
+plugins=(fnox)
+
+# with mise integration
+plugins=(mise fnox)
+```
+
+## Usage
+
+See the [fnox documentation](https://github.com/jdx/fnox#table-of-contents) for information on how to use `fnox` [standalone](https://fnox.jdx.dev/guide/shell-integration.html) or with [`mise` integration](https://fnox.jdx.dev/guide/mise-integration.html).

plugins/fnox/fnox.plugin.zsh

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+if (( ! $+commands[fnox] )); then
+  # if we don't have fnox at all just bail out
+  return
+fi
+
+if (( ! $+commands[mise] )); then
+  # if we have fnox but not mise, source its activation directly
+  eval "$(fnox activate zsh)"
+fi
+
+# If the completion file doesn't exist yet, we need to autoload it and
+# bind it to `mise`. Otherwise, compinit will have already done that.
+if [[ ! -f "$ZSH_CACHE_DIR/completions/_fnox" ]]; then
+  typeset -g -A _comps
+  autoload -Uz _fnox
+  _comps[fnox]=_fnox
+fi
+
+# Generate and load completions
+fnox completion zsh >| "$ZSH_CACHE_DIR/completions/_fnox" &|