feat(planning): start_planning hands back the active plan that already covers the work (#4079)

Step 4 of milestone 415 "An existing plan is found before a new one is made".
A session that could not see an existing plan made a second one beside it.
start_planning and create_milestone now ask first: an ACTIVE milestone in the
project with the same title, or one that reads as the same plan (title, design
and steps against milestone embeddings), is returned with its progress and a
pointer to create_records(milestone_id=...). Nothing is created; force=true
bypasses.

- dedup.find_matching_plan / plan_gate / plan_match_response; access-checked
  before either arm (rule 78), fail-open like the other gates.
- Done milestones never block; the semantic arm needs 200+ chars of candidate.
- kb_plan_match_threshold (default 0.90) is a setting, in the Settings view,
  and pinned against the Python default by 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

@@ -104,6 +104,8 @@ const kbPromptRuleThreshold = ref("0.72");
 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 savingKbInject = ref(false);
 const kbInjectSaved = ref(false);
 
@@ -155,6 +157,9 @@ async function saveKbInject() {
   const dupSnip = Math.min(1, Math.max(0, Number(kbDupThresholdSnippet.value) || 0.82));
   const dupNote = Math.min(1, Math.max(0, Number(kbDupThresholdNote.value) || 0.93));
   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));
   // 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));
@@ -171,6 +176,7 @@ async function saveKbInject() {
   kbDupThresholdSnippet.value = String(dupSnip);
   kbDupThresholdNote.value = String(dupNote);
   kbDupThresholdTask.value = String(dupTask);
+  kbPlanMatchThreshold.value = String(planT);
   kbWritePathThreshold.value = String(wpT);
   kbRuleHintThreshold.value = String(rhT);
   kbToolRuleThreshold.value = String(trT);
@@ -199,6 +205,7 @@ async function saveKbInject() {
       kb_duplicate_threshold_snippet: String(dupSnip),
       kb_duplicate_threshold_note: String(dupNote),
       kb_duplicate_threshold_task: String(dupTask),
+      kb_plan_match_threshold: String(planT),
     });
     kbInjectSaved.value = true;
     setTimeout(() => (kbInjectSaved.value = false), 2000);
@@ -662,6 +669,9 @@ onMounted(async () => {
   if (allSettings.kb_duplicate_threshold_task !== undefined) {
     kbDupThresholdTask.value = allSettings.kb_duplicate_threshold_task;
   }
+  if (allSettings.kb_plan_match_threshold !== undefined) {
+    kbPlanMatchThreshold.value = allSettings.kb_plan_match_threshold;
+  }
   if (allSettings.notify_task_reminders !== undefined) {
     notifyTaskReminders.value = allSettings.notify_task_reminders !== "false";
   }
@@ -1623,6 +1633,27 @@ async function deleteUser(userId: number) {
             stays strict to keep the report pointed at work opened twice.
           </p>
         </div>
+
+        <div class="field">
+          <label for="kb-plan-match-threshold">Existing-plan match threshold</label>
+          <input
+            id="kb-plan-match-threshold"
+            v-model="kbPlanMatchThreshold"
+            type="number"
+            min="0"
+            max="1"
+            step="0.01"
+            class="fs-input input"
+            style="max-width: 8rem"
+          />
+          <p class="field-hint">
+            How alike a new plan must be to an active milestone in the same project
+            before an agent is handed that milestone instead of creating a second
+            one. A plan with the same title always matches. Lower it if sessions
+            still open parallel plans for work that already has one; raise it if
+            they are sent to plans that are only related.
+          </p>
+        </div>
         <div class="actions">
           <button class="btn-primary" @click="saveKbInject" :disabled="savingKbInject">
             {{ savingKbInject ? 'Saving…' : 'Save' }}