Rebuild dotfiles as one branch with per-host layers

Replaces the previous repo, which had split into two histories that never
met (mainframe on a dead GitLab remote, the laptops on Gitea) with 146
dirty files across three machines and the NAS never enrolled at all.

Branch-per-machine is gone. One main, with host differences expressed as
small files under hosts/<hostname>/ rather than as branches, so there is
nothing to merge. The reconciled zsh layer reduces 15-33 line forks to
1-7 effective lines per host; distro differences (oh-my-zsh prefix,
syntax-highlighting path, fd vs fdfind) are probed in common/ instead.

Fresh history: the old one carried six plaintext credentials, 45 MB of
mail caches, browser caches and vendored binaries. 5,096 tracked files
and 144 MB become 462 files and 2.6 MB. The .gitignore is now an
allowlist, which is what keeps that true.

Root cause of the rot: ~/.local/bin was a symlink to scripts/ with GOPATH
inside it, so every go install wrote into version control (2.2 GB on the
work laptop). PATH now points at the repo instead of the reverse.

Also: Hyprland replaces sway and is sourced in two halves so $browser is
defined before use; singleton automations carry ConditionHost= alongside
host-layer-only placement; ddns moves from cron to a guarded timer;
package manifests and pkg-snapshot/pkg-restore replace the X11-era
install_scripts/; networkmanager-dmenu added to system76 (the binding
always existed, the package never did).

hosts/mainframe/systemd/rent-utilities.service

@@ -0,0 +1,22 @@
+[Unit]
+# Singleton: this automation must run on mainframe and nowhere else.
+# Structural guard is hosts/mainframe/ placement; this is the backstop.
+ConditionHost=mainframe
+Description=Assemble the rental utilities message and push it to ntfy
+Documentation=file:///home/connor/docs/leases/deploy/README.md
+# Proton Bridge is a container, not a user unit, so this cannot Requires= it.
+# The script pushes a failure notification if Bridge is down or unauthenticated,
+# which is the point -- a monthly job that fails silently is invisible until the
+# next run, by which time you have stopped expecting it.
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=oneshot
+# Holds PROTON_BRIDGE_PASSWORD. `-` so the unit still runs (and still pushes a
+# useful error) before Bridge has ever been set up.
+EnvironmentFile=-%h/.config/proton/bridge.env
+ExecStart=%h/.dotfiles/hosts/mainframe/bin/rent_utilities
+# Fetching two statements over loopback IMAP; a minute is already generous.
+TimeoutStartSec=300
+Nice=10