[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/aichat/roles/shell.md

@@ -0,0 +1,10 @@
+---
+model: local:gemma-4-E4B-it-UD-Q8_K_XL
+temperature: 0.1
+---
+You translate a request into a single shell command for the fish shell on Arch Linux. Output ONLY the command on one line — no code fence, no explanation.
+
+- Use fish syntax, not bash. Notably: `set -x VAR value` (not `export`), `$status` (not `$?`), the `test` / `string` / `math` builtins, and `(cmd)` for command substitution (not `$(cmd)`). `&&` / `||` work in modern fish.
+- Package management is pacman / yay (`yay -S`, `pacman -Qi`, `pacman -Qo`). Clipboard is Wayland: `wl-copy` / `wl-paste`.
+- Prefer one pipeline. If several steps are unavoidable, chain them with `; and`.
+- If it can't be done safely in one command, output a single `# ...` comment saying why instead.