fix(consult): drop traces, budget checkpoints, silent summary inherit

BUG-729: dropped history rounds leave an omission marker in the model-visible summary slot.
BUG-730: checkpoint threshold is 0.4 of historyBudgetChars (128k still 16,000).
BUG-731: session_full new chat copies owned context_summary on the server; clients send only continued_from_session_id.

frontend/tests/session-context-summary.test.ts

@@ -3,13 +3,14 @@ import { readFileSync } from "node:fs";
 import test from "node:test";
 
 import {
-  CONSULTATION_HISTORY_TAIL_MAX_CHARS,
+  consultationHistoryCheckpointChars,
 } from "../src/lib/consultation-session-history.ts";
 import {
   buildSummaryPrompt,
   checkpointSessionContextSummary,
   generateSessionContextSummary,
   messagesForSummaryInput,
+  resolveInheritedContextSummary,
   sanitizeSessionContextSummary,
   shouldCheckpoint,
   writeSessionContextSummary,
@@ -24,10 +25,14 @@ function overBudgetConversation() {
   ];
 }
 
-test("checkpoint triggers only when the tail exceeds 16_000 characters", () => {
-  assert.equal(CONSULTATION_HISTORY_TAIL_MAX_CHARS, 16_000);
+test("checkpoint triggers only when the tail exceeds the derived threshold", () => {
+  // Former value: hard-coded CONSULTATION_HISTORY_TAIL_MAX_CHARS = 16_000.
+  // 128k still checkpoints at 16_000 (0.4 × 40_000 budget).
+  assert.equal(consultationHistoryCheckpointChars(128_000), 16_000);
   assert.equal(shouldCheckpoint([{ role: "user", text: "x".repeat(15_999) }], null), false);
   assert.equal(shouldCheckpoint([{ role: "user", text: "x".repeat(16_001) }], null), true);
+  assert.equal(shouldCheckpoint([{ role: "user", text: "x".repeat(2_400) }], null, { contextWindow: 64_000 }), false);
+  assert.equal(shouldCheckpoint([{ role: "user", text: "x".repeat(2_401) }], null, { contextWindow: 64_000 }), true);
 });
 
 test("checkpoint prompt omits the last question-answer pair", () => {
@@ -108,6 +113,41 @@ test("writeSessionContextSummary abandons when updatedAt does not match", async
   assert.equal(result, "abandoned");
 });
 
+test("inherited context summary copies owned text and skips foreign or empty sources", async () => {
+  const source = {
+    version: 1 as const,
+    text: "已问过的问题\n事业时机",
+    throughRequestId: "a1",
+    throughMessageIndex: 3,
+    messageCount: 4,
+    updatedAt: "2026-09-15T00:00:00.000Z",
+  };
+  const copied = await resolveInheritedContextSummary({
+    continuedFromSessionId: "11111111-1111-4111-8111-111111111111",
+    loadOwnedSummary: async () => source,
+  });
+  assert.deepEqual(copied, source);
+
+  const foreign = await resolveInheritedContextSummary({
+    continuedFromSessionId: "22222222-2222-4222-8222-222222222222",
+    loadOwnedSummary: async () => null,
+  });
+  assert.equal(foreign, null);
+
+  const empty = await resolveInheritedContextSummary({
+    continuedFromSessionId: "11111111-1111-4111-8111-111111111111",
+    loadOwnedSummary: async () => ({ version: 1, text: "   " }),
+  });
+  assert.equal(empty, null);
+
+  const skipped = await resolveInheritedContextSummary({
+    loadOwnedSummary: async () => {
+      throw new Error("should not load");
+    },
+  });
+  assert.equal(skipped, null);
+});
+
 test("checkpoint writes a new summary when the tail is over budget", async () => {
   const result = await checkpointSessionContextSummary({
     messages: overBudgetConversation(),