deploy: mirror prod env override to test (align environments)

test had no user/env/<host>/ tree because deploy/env/test/ never
existed, so `make remote-apply-env-test` hit its missing-source guard
and was silently skipped — test ran with twig cache off and without the
gzip/Content-Encoding fix, hiding the exact class of gotcha that only
surfaced in prod.

Make deploy/env/test/system.yaml a symlink to deploy/env/prod/system.yaml
so both deployed hosts share one override and can never drift; genericize
the prod file header to say it now serves test + prod.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Np4cMQLF77i664CAQXySzU

deploy/env/prod/system.yaml

@@ -1,4 +1,8 @@
-# Production-only Grav config overrides.
+# Deployed-environment Grav config overrides (test AND prod).
+#
+# Both server environments share this one file so test stays a faithful dress
+# rehearsal of prod: deploy/env/test/system.yaml is a symlink to this file.
+# Edit here and both environments move together — never let them drift.
 #
 # Deep-merged OVER the committed user/config/system.yaml via Grav's
 # per-environment config mechanism: on the server this file is deployed to
@@ -8,9 +12,10 @@
 #
 # These values are deliberately NOT in the committed system.yaml because they
 # would break local development (see CLAUDE.md §1 — dev keeps twig.cache:false
-# so theme edits take effect immediately). Prod is the only place they apply.
+# so theme edits take effect immediately). They apply only on the deployed
+# hosts, never on a local dev checkout.
 #
-# Deploy with:  make remote-apply-env-prod
+# Deploy with:  make remote-apply-env-test  /  make remote-apply-env-prod
 # The user/env/ tree is outside the content repo's tracked folders, so it is
 # NOT restored by content-push / git-sync / remote-fetch-content — re-run the
 # target above after any fresh install.

deploy/env/test/system.yaml

@@ -0,0 +1 @@
+../prod/system.yaml