[Sync] replace the flat layout with the unified stow tree

Supersedes the old flat .config/ layout (last published 2026-06-28) with the
private repo's structure: one shared base plus per-host overlays.

- packages: common/ gui/ lw/ fl/ wm/ plus install.sh and bin/ tooling
  (dotsync, reconcile-hyde.sh)
- new README covering the layout, deploy order and the HyDE dependency
- current HyDE waybar rig (layouts/, cava), pi agent extensions, claude/
  config, tmux, presenterm, aichat roles
- fish: kp (keepassxc-cli + fzf picker, db path from $KP_DB) and
  bind_M_n_history (alt+1..9 recalls the nth history entry)
- drops cruft that should never have been tracked: the duplicate top-level
  .pi/ copy, btop.log, zellij config.kdl.bak, fish_variables
- .pi/agent/auth.json is gitignored; auth.json.example ships instead

Host-specific work sessions and the personal backlog stay in the private
tree. Endpoint locators in the llamacpp/whisper guides are placeholders
($SERVER, <own-domain>) — the guides themselves stay, since they are the
useful part.

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/" "")'