[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/tmux/scripts/mem.sh

@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+# RAM utilisation (%) — used = MemTotal - MemAvailable, from /proc/meminfo.
+# Called from tmux status-right via #(); cached per status-interval.
+printf '\U000f0f86 '   # Waybar's memory glyph (U+F0F86), then the percentage
+awk '/^MemTotal:/{t=$2} /^MemAvailable:/{a=$2} END{if(t>0)printf "%d%%",(t-a)*100/t; else printf "--%%"}' /proc/meminfo