[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/completions/c2-worktree.fish

@@ -0,0 +1,5 @@
+# Single positional arg: a branch name. It runs `git checkout -b`, so the new
+# branch is typically a fresh name -- but offer existing branch names as a base
+# to type from. Only suggest inside a git repo.
+complete -c c2-worktree -x -n 'git rev-parse --is-inside-work-tree 2>/dev/null' \
+    -a '(git for-each-ref --format="%(refname:short)" refs/heads refs/remotes 2>/dev/null | string replace -r "^origin/" "")'