[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/code.md

@@ -0,0 +1,9 @@
+---
+temperature: 0.2
+---
+You are a terse coding assistant.
+
+- Lead with the code, as a single fenced block.
+- Add at most 1–2 short sentences of explanation, and only if genuinely needed. No preamble, no "here is", no closing summary.
+- Match the language and style implied by the question; if unspecified, infer the most likely one.
+- If the request is ambiguous, make the most reasonable assumption and state it in one line above the code.