FN-5752: flush merger agent logs before disposing sessions

Ensure merger AI activity is persisted to task logs before sessions are torn down.

- Flush AgentLogger output in merger AI-session finally blocks for autostash conflict resolution, autostash hard-fail recovery, and rebase conflict resolution paths
- Guard flush/dispose cleanup with safe try/catch handling to avoid teardown-time logging loss
- Add a regression test asserting logger flush occurs before session disposal during AI rebase conflict resolution
- Export the rebase conflict resolver helper for targeted test coverage
- Add a patch changeset for @runfusion/fusion documenting the agent-log visibility fix

Files changed:
 .changeset/fn-5752-merger-log-flush.md             |  5 ++++
 packages/engine/src/__tests__/merger-conflict-resolution.test.ts   | 30 ++++++++++++++++++++++
 packages/engine/src/merger.ts                      | 22 +++++++++++++++-
 3 files changed, 56 insertions(+), 1 deletion(-)

Fusion-Task-Id: FN-5752

Fusion-Task-Lineage: 3c6023f0-cc9b-47e1-bfd4-69b40e011d84

.changeset/fn-5752-merger-log-flush.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": patch
+---
+
+Fix merger agent-log visibility by flushing buffered `AgentLogger` output before disposing AI sessions used for autostash conflict resolution, autostash hard-fail recovery, and rebase conflict resolution. This ensures trailing text/thinking deltas are persisted so merger activity reliably appears in the task agent log panel.

packages/engine/src/__tests__/merger-conflict-resolution.test.ts

@@ -145,8 +145,10 @@ import {
   resolveTaskDiffBaseRef,
   commitOrAmendMergeWithFixes,
   MergeAbortedError,
+  __testOnlyResolveComplexRebaseConflictsWithAi,
   type ConflictCategory,
 } from "../merger.js";
+import { AgentLogger } from "../agent-logger.js";
 import { mergerLog } from "../logger.js";
 import { createFnAgent } from "../pi.js";
 import { execSync, exec } from "node:child_process";
@@ -266,6 +268,34 @@ function setupFailingFallbackStrategy() {
 /** @deprecated Renamed to setupFailingFallbackStrategy. */
 const setupFailingTheirsStrategy = setupFailingFallbackStrategy;
 
+describe("merger agent logger flushing", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("flushes buffered rebase conflict agent logs before disposing session", async () => {
+    const session = {
+      prompt: vi.fn().mockResolvedValue(undefined),
+      dispose: vi.fn(),
+      getSessionStats: vi.fn().mockReturnValue(undefined),
+    };
+    mockedCreateFnAgent.mockResolvedValue({ session } as any);
+
+    const flushSpy = vi.spyOn(AgentLogger.prototype, "flush").mockResolvedValue(undefined);
+
+    await __testOnlyResolveComplexRebaseConflictsWithAi(
+      createMockStore(),
+      "/tmp/root",
+      "FN-5752",
+      { ...DEFAULT_SETTINGS },
+      ["src/conflicted.ts"],
+    );
+
+    expect(flushSpy).toHaveBeenCalledTimes(1);
+    expect(session.dispose).toHaveBeenCalledTimes(1);
+    expect(flushSpy.mock.invocationCallOrder[0]).toBeLessThan(session.dispose.mock.invocationCallOrder[0]);
+  });
+});
 
 describe("detectResolvableConflicts", () => {
   beforeEach(() => {

packages/engine/src/merger.ts

@@ -3054,6 +3054,11 @@ ${fileList}
     await store.logEntry(taskId, "Autostash conflict agent encountered an error", msg);
     return { success: false, error: msg };
   } finally {
+    try {
+      await agentLogger.flush();
+    } catch {
+      // ignore
+    }
     try {
       session.dispose();
     } catch {
@@ -3465,6 +3470,11 @@ ${fileList}
     await store.logEntry(taskId, "Autostash hard-fail recovery agent encountered an error", msg);
     return { success: false, error: msg };
   } finally {
+    try {
+      await agentLogger.flush();
+    } catch {
+      // ignore
+    }
     try {
       session.dispose();
     } catch {
@@ -3983,6 +3993,7 @@ async function buildDeterministicMergeMessage(params: {
 }
 
 export { buildDeterministicMergeMessage as __testOnlyBuildDeterministicMergeMessage };
+export { resolveComplexRebaseConflictsWithAi as __testOnlyResolveComplexRebaseConflictsWithAi };
 
 /**
  * Stage current changes and either:
@@ -6826,7 +6837,16 @@ You are assisting with a paused \`git pull --rebase\`.
     });
     await accumulateSessionTokenUsage(store, taskId, session);
   } finally {
-    session.dispose();
+    try {
+      await agentLogger.flush();
+    } catch {
+      // ignore
+    }
+    try {
+      session.dispose();
+    } catch {
+      // ignore
+    }
   }
 }