[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/fish/functions/__ssh_agent_is_started.fish

@@ -0,0 +1,23 @@
+function __ssh_agent_is_started -d "check if ssh agent is already started"
+	if test -n "$SSH_CONNECTION"
+		# This is an SSH session
+		ssh-add -l > /dev/null 2>&1
+		if test $status -eq 0 -o $status -eq 1
+			# An SSH agent was forwarded
+			return 0
+		end
+	end
+
+	if begin; test -f "$SSH_ENV"; and test -z "$SSH_AGENT_PID"; end
+		source $SSH_ENV > /dev/null
+	end
+
+	if test -z "$SSH_AGENT_PID"
+		return 1
+	end
+
+	ssh-add -l > /dev/null 2>&1
+	if test $status -eq 2
+		return 1
+	end
+end