[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/.pi/agent/prompts/commit.md

@@ -0,0 +1,11 @@
+---
+description: Write a Conventional Commits message for the staged changes
+argument-hint: "[extra context]"
+---
+Inspect the staged changes with `git diff --cached` (and `git status` for context). Then write a single Conventional Commits message:
+
+- A `<type>(<scope>): <subject>` summary line, imperative mood, ≤72 chars (types: feat, fix, refactor, docs, chore, test, perf, build).
+- An optional short body explaining the *why* only when it isn't obvious from the diff.
+- Do NOT run `git commit`. Output only the message inside a code block so I can review it.
+
+Extra context to incorporate: ${1:-none}