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/zellij.plugin.zsh

@@ -0,0 +1,35 @@
+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" &|