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>

dot_config/systemd/user/omp-loop@.service

@@ -0,0 +1,10 @@
+[Unit]
+Description=omp agent loop: %i
+
+[Service]
+Type=oneshot
+ExecStart=%h/.omp/loops/bin/run-loop.sh %i
+# Loops are green (read-only + own state file) but belt-and-braces:
+# no interactive session, hard runtime ceiling above the script's own caps.
+TimeoutStartSec=45min
+Nice=10