build: per-environment Grav config override (prod Twig prod-mode)

Prod needs twig.cache:true / debug:false / auto_reload:false, but those
values break local dev, so they must not live in the committed
system.yaml. Add a per-environment override deployed only to the server
via Grav's environment://config (user/env/<host>/config/system.yaml):

- deploy/env/prod/system.yaml — version-controlled source of truth.
- make remote-apply-env-prod — writes it to the server + clears cache;
  resolves the host in-recipe (WEB_HOST || REMOTE_HOST) to avoid the
  recursive-make empty-export trap.
- remote-diag now shows the deployed override + whether twig cache is
  populating, so prod-mode can be verified not assumed.
- CLAUDE.md §1 rewritten: never flip committed system.yaml; use the
  override. Backlog updated (twig prod-mode + /post login-gate done;
  note stale .env.prod GRAV_VERSION and pending git-sync).

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

deploy/env/prod/system.yaml

@@ -0,0 +1,20 @@
+# Production-only Grav config overrides.
+#
+# Deep-merged OVER the committed user/config/system.yaml via Grav's
+# per-environment config mechanism: on the server this file is deployed to
+#   <webroot>/user/env/<hostname>/config/system.yaml
+# and Grav's `environment://config` stream (keyed on the request hostname)
+# layers it on top of `user://config`.
+#
+# 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.
+#
+# Deploy with:  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.
+twig:
+  cache: true
+  debug: false
+  auto_reload: false