feat(arch-layer,scripts): linux-gated desktop configs + package install scripts

niri (exact_config.d, scripts), noctalia (plugins excluded — runtime
state with embedded token), fontconfig, mimeapps.list,
niri-dynamic-workspaces, systemd user unit files (enable-state not
managed; only nirinit enabled by run_once), niri-launch vendored from
/usr/bin (unowned hand-written script, shellcheck warnings pre-existing).

Scripts: packages (pacman --needed + paru, brew on darwin, root-safe,
keyed on packages.yaml hash), gh extensions (idempotent), user services
(linux), post (tpm clone, bat cache, fc-cache). All shellcheck-clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.chezmoiscripts/run_once_after_30-user-services.sh.tmpl

@@ -0,0 +1,15 @@
+{{ if eq .chezmoi.os "linux" -}}
+#!/bin/sh
+# Enable session-critical user services. Personal automation units
+# (brain-lens, omp-loop, rclone, hermes, voxtype) ship as files only —
+# enable manually per machine. See MIGRATION-NOTES.md.
+set -eu
+
+if [ -f "$HOME/.config/systemd/user/nirinit.service" ] && command -v systemctl >/dev/null 2>&1; then
+    systemctl --user daemon-reload || true
+    systemctl --user enable nirinit.service || true
+fi
+{{ else -}}
+#!/bin/sh
+exit 0
+{{ end -}}