fix(planning): the plan gate's default drops to 0.80, where reworded plans actually score (#4079)

Measured live after deploy: three plans reworded from existing active
milestones scored 0.83-0.87 against the milestone they restated, and the
nearest distinct plans 0.72-0.77. At the inherited 0.90 the semantic arm
matched nothing, so only an identical title was caught. 0.80 sits in the gap.
The Settings form's default moves with it (test_settings_defaults_agree).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

frontend/src/views/SettingsView.vue

@@ -105,7 +105,7 @@ const kbDupThresholdSnippet = ref("0.82");
 const kbDupThresholdNote = ref("0.93");
 const kbDupThresholdTask = ref("0.93");
 // PLAN_MATCH_DEFAULT_THRESHOLD in services/dedup.py.
-const kbPlanMatchThreshold = ref("0.90");
+const kbPlanMatchThreshold = ref("0.80");
 const savingKbInject = ref(false);
 const kbInjectSaved = ref(false);
 
@@ -159,7 +159,7 @@ async function saveKbInject() {
   const dupTask = Math.min(1, Math.max(0, Number(kbDupThresholdTask.value) || 0.93));
   // Same `|| default` guard: a floor of 0 would hand back an existing plan
   // for every new one, and no plan could be started without force.
-  const planT = Math.min(1, Math.max(0, Number(kbPlanMatchThreshold.value) || 0.9));
+  const planT = Math.min(1, Math.max(0, Number(kbPlanMatchThreshold.value) || 0.8));
   // Same `|| default` reasoning: falling back to 0 would surface every
   // snippet in the corpus on every edit, which is the failure this knob fixes.
   const wpT = Math.min(1, Math.max(0, Number(kbWritePathThreshold.value) || 0.68));