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>

agent/BACKLOG.md

@@ -26,10 +26,11 @@ next, in what order*.
 ROADMAP § Full docs review. Done: the roadmap/backlog split (this file);
 slice (a) option-surface reconcile — README tables fixed (9 gaps) and
 the diff made permanent as the `option-docs` flake check (eval-walked
-surface vs tables, catches stale rows too). Remaining slices:
-- **(b) drift pass** over docs/OVERRIDES.md, docs/TESTING.md,
-  templates/downstream/README.md — read each against the current tree,
-  fix what lies.
+surface vs tables, catches stale rows too); slice (b) drift pass over
+docs/OVERRIDES.md, docs/TESTING.md, templates/downstream/README.md —
+every factual claim spot-checked against the tree, all held; fixed the
+two gaps (OVERRIDES now routes to `nomarchy.monitors`; TESTING §1 lists
+the shell-syntax check its CI § claimed). Remaining slices:
 - **(c) install/first-run story** read end to end (README install §§ +
   template README as one narrative; fix the seams).
 - **(d) recovery runbook** — short doc: desktop won't start → boot an