feat(zellij): add zellij plugin with aliases and completions

Add a new plugin for the zellij terminal multiplexer providing:
- 9 shorthand aliases with dynamic prefix (z or zj to avoid conflicts)
- Background completion caching using the same pattern as the gh plugin

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

plugins/zellij/README.md

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+# zellij
+
+This plugin provides aliases and completions for [zellij](https://zellij.dev/), the terminal workspace
+(multiplexer). To use it, add `zellij` to the plugins array in your zshrc file.
+
+```zsh
+plugins=(... zellij)
+```
+
+## Dynamic prefix
+
+The default alias prefix is `z`. If `z` is already taken by another plugin (e.g., the
+[suse](https://github.com/ohmyzsh/ohmyzsh/tree/master/plugins/suse) plugin), the prefix
+automatically falls back to `zj`.
+
+## Aliases
+
+| Alias      | Command                      | Description              |
+| ---------- | ---------------------------- | ------------------------ |
+| `z`/`zj`   | `zellij`                     | Zellij command           |
+| `za`/`zja`  | `zellij attach`              | Attach to a session      |
+| `zd`/`zjd`  | `zellij delete-session`      | Delete a session         |
+| `zda`/`zjda` | `zellij delete-all-sessions` | Delete all sessions      |
+| `zk`/`zjk`  | `zellij kill-session`        | Kill a session           |
+| `zka`/`zjka` | `zellij kill-all-sessions`   | Kill all sessions        |
+| `zl`/`zjl`  | `zellij list-sessions`       | List sessions            |
+| `zr`/`zjr`  | `zellij run`                 | Run a command in a pane  |
+| `zs`/`zjs`  | `zellij -s`                  | Start a named session    |
+
+## Completions
+
+This plugin caches the zellij completion script in the background (using the same approach as
+the [gh](https://github.com/ohmyzsh/ohmyzsh/tree/master/plugins/gh) plugin). On first load the
+cache is generated; completions become available in the next shell session.

plugins/zellij/zellij.plugin.zsh

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+if (( ! $+commands[zellij] )); then
+  return
+fi
+
+# Dynamic prefix: use "z" if no conflict, fall back to "zj"
+if (( $+aliases[z] )); then
+  _zellij_prefix="zj"
+else
+  _zellij_prefix="z"
+fi
+
+# Aliases
+alias ${_zellij_prefix}='zellij'
+alias ${_zellij_prefix}a='zellij attach'
+alias ${_zellij_prefix}d='zellij delete-session'
+alias ${_zellij_prefix}da='zellij delete-all-sessions'
+alias ${_zellij_prefix}k='zellij kill-session'
+alias ${_zellij_prefix}ka='zellij kill-all-sessions'
+alias ${_zellij_prefix}l='zellij list-sessions'
+alias ${_zellij_prefix}r='zellij run'
+alias ${_zellij_prefix}s='zellij -s'
+
+unset _zellij_prefix
+
+# Completion caching (same pattern as gh plugin)
+# On first load, _zellij may not exist in fpath — autoload fails silently.
+# The background command generates the cache file for subsequent sessions.
+# Load order: plugin loads → compinit runs → next session picks up cached file.
+if [[ ! -f "$ZSH_CACHE_DIR/completions/_zellij" ]]; then
+  typeset -g -A _comps
+  autoload -Uz _zellij
+  _comps[zellij]=_zellij
+fi
+
+zellij setup --generate-completion zsh >| "$ZSH_CACHE_DIR/completions/_zellij" &|