FN-5911: move agent logs to per-task JSONL storage

Store agent logs in per-task JSONL files instead of the legacy SQLite table.

- add a file-backed agent log store with JSONL append/read/prune helpers and task-scoped source refs
- migrate legacy SQLite agentLogEntries data into task files, rewrite goal citation references, and preserve soft-deleted logs for forensics
- update task store, settings, docs, dashboard route coverage, and add regression tests for migration, retention, and log access

Files changed:
 .changeset/fn-5911-agent-log-jsonl.md              |   5 +
 AGENTS.md                                          |   2 +-
 docs/diagnostics.md                                |   2 +-
 docs/settings-reference.md                         |   2 +
 docs/soft-delete-verification-matrix.md            |   7 +-
 docs/storage.md                                    |   8 +-
 .../src/__tests__/agent-log-file-store.test.ts     | 123 ++++++
 .../core/src/__tests__/agent-log-migration.test.ts | 186 +++++++++
 .../core/src/__tests__/agent-log-retention.test.ts | 208 ++++++++++
 packages/core/src/__tests__/db-migrate.test.ts     |  14 +-
 packages/core/src/__tests__/db.test.ts             |  39 +-
 .../src/__tests__/goal-citations-store.test.ts     |  38 +-
 packages/core/src/__tests__/goals-schema.test.ts   |   2 +-
 packages/core/src/__tests__/insight-store.test.ts  |  10 +-
 .../src/__tests__/merge-request-record.test.ts     |   2 +-
 packages/core/src/__tests__/mission-store.test.ts  |   2 +-
 packages/core/src/__tests__/run-audit.test.ts      |   2 +-
 packages/core/src/__tests__/secrets-schema.test.ts |   6 +-
 .../src/__tests__/soft-delete-agent-logs.test.ts   |  71 ++--
 .../src/__tests__/store-agent-log-file.test.ts     |  91 +++++
 .../core/src/__tests__/store-merge-queue.test.ts   |   2 +-
 packages/core/src/__tests__/store-test-helpers.ts  |  43 +-
 packages/core/src/__tests__/store-upsert.test.ts   |  37 +-
 packages/core/src/__tests__/task-documents.test.ts |   2 +-
 packages/core/src/agent-log-constants.ts           |  25 ++
 packages/core/src/agent-log-file-store.ts          | 267 ++++++++++++
 packages/core/src/db.ts                            |  51 ++-
 packages/core/src/settings-schema.ts               |   1 +
 packages/core/src/store.ts                         | 453 ++++++++++++---------
 packages/core/src/types.ts                         |  10 +-
 .../__tests__/agent-log-routes.integration.test.ts |  48 +++
 .../src/__tests__/evaluator-evidence.test.ts       |  47 ++-
 packages/engine/src/self-healing.ts                |  12 +
 .../src/store/__tests__/roadmap-store.test.ts      |   4 +-
 34 files changed, 1477 insertions(+), 345 deletions(-)

Fusion-Task-Id: FN-5911

Fusion-Task-Lineage: 07c42f3a-87cf-4558-8f01-ac8460b5558b

packages/dashboard/src/__tests__/agent-log-routes.integration.test.ts

@@ -0,0 +1,48 @@
+// @vitest-environment node
+
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import express from "express";
+import { existsSync, mkdtempSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { TaskStore } from "@fusion/core";
+import { createApiRoutes } from "../routes.js";
+import { get } from "../test-request.js";
+
+describe("task log routes with file-backed agent logs", () => {
+  let rootDir: string;
+  let store: TaskStore;
+  let app: express.Express;
+
+  beforeEach(async () => {
+    rootDir = mkdtempSync(join(tmpdir(), "fusion-dashboard-agent-log-routes-"));
+    store = new TaskStore(rootDir, join(rootDir, ".fusion-global-settings"), { inMemoryDb: true });
+    await store.init();
+    app = express();
+    app.use(express.json());
+    app.use("/api", createApiRoutes(store));
+  });
+
+  afterEach(() => {
+    store.close();
+    rmSync(rootDir, { recursive: true, force: true });
+  });
+
+  it("GET /api/tasks/:id/logs returns file-backed entries with count headers", async () => {
+    const task = await store.createTask({ description: "Route reads file-backed agent logs" });
+    await store.appendAgentLog(task.id, "first", "text", undefined, "executor");
+    await store.appendAgentLog(task.id, "second", "tool", "detail-2", "executor");
+    await store.appendAgentLog(task.id, "third", "tool_result", "detail-3", "executor");
+
+    const expected = await store.getAgentLogs(task.id, { limit: 2 });
+    const agentLogPath = join(rootDir, ".fusion", "tasks", task.id, "agent-log.jsonl");
+    expect(existsSync(agentLogPath)).toBe(true);
+
+    const res = await get(app, `/api/tasks/${task.id}/logs?limit=2`);
+
+    expect(res.status).toBe(200);
+    expect(res.body).toEqual(expected);
+    expect(res.headers["x-total-count"]).toBe("3");
+    expect(res.headers["x-has-more"]).toBe("true");
+  });
+});