feat(zsh): alias tree -> erdtree, track its config, install it on fresh boxes

erdtree (erd) replaces GNU tree as the tree-style directory lister. It uses
a parallel walker (the `ignore` crate, one thread per core) where GNU tree is
strictly serial. Benchmarked over 658k entries under ~/development, best-of-3,
warm cache, plain output, hidden included, gitignore off:

  erd (16 threads)   2.35s
  GNU tree 2.1.0     4.22s   (1.8x slower)
  erd (1 thread)     4.97s   (so ~2.1x thread scaling)
  eza --tree        15.87s   (6.8x slower)

Three changes:

- home_config/erdtree/.erdtree.toml — erd ships as a disk-usage tool, so its
  stock defaults are root-at-bottom, size-sorted, with a du column. The
  top-level table overrides that to behave like GNU tree; `erd -c du` keeps
  the original behaviour and `erd -c all` disables the gitignore/hidden
  filtering. Stowed to ~/.config/erdtree.

- home_root/.zshrc — `alias tree='erd'`, plus ~/.zfunc prepended to fpath for
  the generated completion. The fpath line goes before the primary compinit
  at the top of the file rather than adding a third compinit call after it.

- lk-installers/install-apps — erd is now a runtime assumption of the
  dotfiles (the alias depends on it), so a fresh box must get it: brew on
  macOS, cargo on Linux (not packaged in Debian apt). The zsh completion is
  generated per installed version rather than tracked, matching how the other
  generated completions in this repo are handled.

Caveat worth knowing: erd respects .gitignore by default, which GNU tree does
not. `-i` disables it, `-.` shows hidden files.

lk-installers/install-apps

@@ -15,6 +15,16 @@
 # install-zellij scripts.
 set -euo pipefail
 
+# erdtree (`erd`) — .zshrc aliases `tree` to it, so it is a runtime assumption
+# of the dotfiles, not an optional extra. Its zsh completion is generated per
+# installed version rather than tracked in the repo; .zshrc prepends ~/.zfunc
+# to fpath to pick it up.
+generate_erd_completion() {
+  command -v erd >/dev/null 2>&1 || return 0
+  mkdir -p "$HOME/.zfunc"
+  erd --completions zsh > "$HOME/.zfunc/_erd"
+}
+
 # fzf-git.sh is a sourced helper (not a package) — same on every platform.
 clone_fzf_git() {
   if [ -d "$HOME/fzf-git.sh" ]; then
@@ -47,6 +57,11 @@ install_macos() {
   # owns that file).
   "$(brew --prefix)/opt/fzf/install" --key-bindings --completion --no-update-rc >/dev/null
 
+  # erdtree: brew formula, cargo as fallback if the formula ever disappears.
+  command -v erd >/dev/null 2>&1 || brew install erdtree || cargo install erdtree \
+    || echo "erdtree unavailable — the \`tree\` alias will not work"
+  generate_erd_completion
+
   clone_fzf_git
 }
 
@@ -89,6 +104,11 @@ install_linux() {
 
   # zellij: cargo build, fall back to apt
   command -v zellij >/dev/null 2>&1 || cargo install zellij || sudo apt install -y zellij
+
+  # erdtree: not packaged in Debian apt, so cargo only (apt installs cargo above).
+  command -v erd >/dev/null 2>&1 || cargo install erdtree \
+    || echo "erdtree unavailable — the \`tree\` alias will not work"
+  generate_erd_completion
 }
 
 case "$(uname -s)" in