fix(setup,config): harden plan-tune decision against bad input

Review follow-ups to the non-interactive plan-tune work:
- setup now lowercases + whitespace-strips the resolved decision before the
  case match, so an explicit opt-in via flag/env ("YES", "Yes", " yes") is
  honored instead of silently falling through to "prompt"/skip. Also accepts
  on/off and 1/0.
- gstack-config rejects out-of-domain plan_tune_hooks values (anything but
  prompt|yes|no) with a warning + fallback to prompt, matching the existing
  value-whitelist pattern for explain_level / artifacts_sync_mode.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

bin/gstack-config

@@ -298,6 +298,10 @@ case "${1:-}" in
       echo "Warning: redact_prepush_hook '$VALUE' not recognized. Valid values: true, false. Using false." >&2
       VALUE="false"
     fi
+    if [ "$KEY" = "plan_tune_hooks" ] && [ "$VALUE" != "prompt" ] && [ "$VALUE" != "yes" ] && [ "$VALUE" != "no" ]; then
+      echo "Warning: plan_tune_hooks '$VALUE' not recognized. Valid values: prompt, yes, no. Using prompt." >&2
+      VALUE="prompt"
+    fi
     mkdir -p "$STATE_DIR"
     # Write annotated header on first creation
     if [ ! -f "$CONFIG_FILE" ]; then