docs(review): drift pass over OVERRIDES/TESTING/template README (item 6b)

Slice (b) of the docs review: every factual claim in the three docs
spot-checked against the tree (mkDefault sites, bind priority, CLI
keys, awww naming, preset/wallpaper counts, summer-night's light bar,
live-ISO monitor rule, CI workflow, QEMU GL flags, template steps) —
all held. Two gaps fixed: OVERRIDES §2 predated nomarchy.monitors and
never routed readers to it (added, plus a quick-reference row);
TESTING §1 didn't list the shell-syntax check that §1b says CI runs
(added bash -n).

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

docs/TESTING.md

@@ -14,6 +14,7 @@ than claiming success. "All Nix files parse" is not "the bar renders."
 nix flake check --no-build          # full module-system evaluation, no builds
 nix-instantiate --parse <file>      # syntax-only, works even on macOS
 python3 -m py_compile pkgs/nomarchy-theme-sync/nomarchy-theme-sync.py
+bash -n <script>.sh                 # shell syntax (tools/, installer bits)
 ```
 
 `nix flake check` needs a Linux machine (or a Linux builder) since all