[Sync] adopt unified stow layout from the private repo

Mirrors the private dots tree at 900bdda: one shared base plus per-host
overlays, replacing the old flat .config/ layout (last synced 2026-06-28).

- packages: common/ gui/ wm/ lw/ fl/ + install.sh and bin/ tooling (dotsync,
  reconcile-hyde.sh)
- new README (layout, deploy order, HyDE dependency), plus ToDo.md and
  HYDE-UPDATE.md
- current HyDE waybar rig (layouts/, cava), pi agent extensions, claude/
  config, tmux, presenterm, aichat roles
- drops stale duplicates and generated cruft that should never have been
  tracked: the second top-level .pi/ copy, btop.log, zellij config.kdl.bak,
  fish_variables, nvim codecompanion.lua
- .pi/agent/auth.json is gitignored now; auth.json.example ships instead
- fl/ and wm/ hypr themes/ stay untracked (HyDE-generated per machine, per
  the root .gitignore)

common/.config/fish/functions/c2-worktree.fish

@@ -0,0 +1,30 @@
+function c2-worktree
+    if test (count $argv) -ne 1
+        echo "Usage: c2-worktree <branch-name>"
+        echo "Example: c2-worktree package/package-update"
+        return 1
+    end
+
+    set branch $argv[1]
+
+    set root (git rev-parse --show-toplevel)
+    or return 1
+    set dir $root/.worktrees/(basename $branch)
+
+    mkdir -p $root/.worktrees
+    or return 1
+
+    git checkout -b $branch
+    or return 1
+
+    git checkout master
+    or return 1
+
+    git worktree add $dir $branch
+    or return 1
+
+    cd $dir
+    or return 1
+
+    pnpm i
+end