test(plan-tune): regression coverage for v1.49 consent + setup gates

Plan-tune cathedral T2 + part of T1 follow-up (Codex IRON RULE — regressions
get tests). v1.49 shipped two prose-driven implicit gates inside plan-tune
Step 0 (consent, setup) with zero test coverage. The cathedral refactors that
template heavily; without tests, silent breakage is possible.

Three regression families plus a static template assertion:
1. Consent gate fires under qt=false + no marker; goes silent on marker write
   or qt=true flip.
2. Setup gate fires under qt=true + empty declared + no marker; goes silent
   when declared populates, marker is written, or qt is still false.
3. Marker idempotency: gates stay silent across 5 re-invocations after a
   single decline/bail. Markers honored independently.
4. Static template assertion: gate language can't be silently deleted
   without breaking a test.

Also extends gstack-config to honor GSTACK_STATE_ROOT (it was the last bin
still ignoring it — caught while writing the tests; without this, tests
would silently mutate the user's real config.yaml).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

bin/gstack-config

@@ -8,11 +8,13 @@
 #   gstack-config defaults           — show just the defaults table
 #
 # Env overrides (for testing):
+#   GSTACK_STATE_ROOT — override ~/.gstack state directory (highest priority,
+#                       matches D16 cathedral isolation convention)
 #   GSTACK_HOME       — override ~/.gstack state directory (aligns with writer scripts)
 #   GSTACK_STATE_DIR  — legacy alias for GSTACK_HOME (kept for backwards compat)
 set -euo pipefail
 
-STATE_DIR="${GSTACK_HOME:-${GSTACK_STATE_DIR:-$HOME/.gstack}}"
+STATE_DIR="${GSTACK_STATE_ROOT:-${GSTACK_HOME:-${GSTACK_STATE_DIR:-$HOME/.gstack}}}"
 CONFIG_FILE="$STATE_DIR/config.yaml"
 
 # Annotated header for new config files. Written once on first `set`.