feat(HAI-072): add agent log viewer with persistence, streaming, and UI

- Add agent log persistence layer with JSONL append/read and event emission in core store
- Add server-side SSE log streaming endpoint and REST route for fetching logs
- Create AgentLogViewer component and useAgentLogs hook for real-time log display
- Integrate log viewer into TaskDetailModal
- Fix pre-existing build and test errors

packages/core/src/index.ts

@@ -1,4 +1,4 @@
 export { COLUMNS, COLUMN_LABELS, COLUMN_DESCRIPTIONS, VALID_TRANSITIONS, DEFAULT_SETTINGS } from "./types.js";
-export type { Column, Task, TaskAttachment, TaskCreateInput, TaskDetail, BoardConfig, MergeResult, Settings, TaskStep, StepStatus, TaskLogEntry } from "./types.js";
+export type { Column, Task, TaskAttachment, TaskCreateInput, TaskDetail, AgentLogEntry, BoardConfig, MergeResult, Settings, TaskStep, StepStatus, TaskLogEntry } from "./types.js";
 export { TaskStore } from "./store.js";
 export { canTransition, getValidTransitions, resolveDependencyOrder } from "./board.js";

packages/core/src/store.test.ts

@@ -400,4 +400,50 @@ describe("TaskStore", () => {
       expect(updated.blockedBy).toBeUndefined();
     });
   });
+
+  describe("agent log persistence", () => {
+    it("appendAgentLog creates agent.log and getAgentLogs reads it back", async () => {
+      const task = await createTestTask();
+
+      await store.appendAgentLog(task.id, "Hello world", "text");
+      await store.appendAgentLog(task.id, "Read", "tool");
+
+      const logs = await store.getAgentLogs(task.id);
+      expect(logs).toHaveLength(2);
+      expect(logs[0].text).toBe("Hello world");
+      expect(logs[0].type).toBe("text");
+      expect(logs[0].taskId).toBe(task.id);
+      expect(logs[1].text).toBe("Read");
+      expect(logs[1].type).toBe("tool");
+    });
+
+    it("getAgentLogs returns empty array when no log file exists", async () => {
+      const task = await createTestTask();
+      const logs = await store.getAgentLogs(task.id);
+      expect(logs).toEqual([]);
+    });
+
+    it("appendAgentLog emits agent:log event", async () => {
+      const task = await createTestTask();
+      const events: any[] = [];
+      store.on("agent:log", (entry) => events.push(entry));
+
+      await store.appendAgentLog(task.id, "delta text", "text");
+
+      expect(events).toHaveLength(1);
+      expect(events[0].text).toBe("delta text");
+      expect(events[0].type).toBe("text");
+      expect(events[0].taskId).toBe(task.id);
+    });
+
+    it("handles multiple appends correctly (JSONL format)", async () => {
+      const task = await createTestTask();
+      for (let i = 0; i < 5; i++) {
+        await store.appendAgentLog(task.id, `chunk ${i}`, "text");
+      }
+      const logs = await store.getAgentLogs(task.id);
+      expect(logs).toHaveLength(5);
+      expect(logs[4].text).toBe("chunk 4");
+    });
+  });
 });

packages/core/src/store.ts

@@ -1,9 +1,9 @@
 import { EventEmitter } from "node:events";
 import { execSync } from "node:child_process";
-import { mkdir, readFile, writeFile, readdir, rename, unlink } from "node:fs/promises";
+import { appendFile, mkdir, readFile, writeFile, readdir, rename, unlink } from "node:fs/promises";
 import { join, sep } from "node:path";
 import { existsSync, watch, type FSWatcher } from "node:fs";
-import type { Task, TaskDetail, TaskCreateInput, TaskAttachment, BoardConfig, Column, MergeResult, Settings } from "./types.js";
+import type { Task, TaskDetail, TaskCreateInput, TaskAttachment, AgentLogEntry, BoardConfig, Column, MergeResult, Settings } from "./types.js";
 import { VALID_TRANSITIONS, DEFAULT_SETTINGS } from "./types.js";
 
 export interface TaskStoreEvents {
@@ -12,6 +12,7 @@ export interface TaskStoreEvents {
   "task:updated": [task: Task];
   "task:deleted": [task: Task];
   "task:merged": [result: MergeResult];
+  "agent:log": [entry: AgentLogEntry];
 }
 
 export class TaskStore extends EventEmitter<TaskStoreEvents> {
@@ -839,6 +840,52 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
     });
   }
 
+  /**
+   * Append an agent log entry to the task's agent log file (JSONL format).
+   * Each entry is a single JSON line appended to `.hai/tasks/{ID}/agent.log`.
+   * Also emits an `agent:log` event for live streaming.
+   *
+   * @param taskId - The task ID (e.g. "HAI-001")
+   * @param text - The text content (delta for "text", tool name for "tool")
+   * @param type - Whether this is a "text" delta or a "tool" invocation marker
+   */
+  async appendAgentLog(taskId: string, text: string, type: "text" | "tool"): Promise<void> {
+    const entry: AgentLogEntry = {
+      timestamp: new Date().toISOString(),
+      taskId,
+      text,
+      type,
+    };
+    const dir = this.taskDir(taskId);
+    const logPath = join(dir, "agent.log");
+    await appendFile(logPath, JSON.stringify(entry) + "\n");
+    this.emit("agent:log", entry);
+  }
+
+  /**
+   * Read all historical agent log entries for a task from its agent log file.
+   * Returns entries in chronological order (oldest first).
+   *
+   * @param taskId - The task ID (e.g. "HAI-001")
+   * @returns Array of agent log entries, empty if no log file exists
+   */
+  async getAgentLogs(taskId: string): Promise<AgentLogEntry[]> {
+    const dir = this.taskDir(taskId);
+    const logPath = join(dir, "agent.log");
+    if (!existsSync(logPath)) return [];
+    const content = await readFile(logPath, "utf-8");
+    const entries: AgentLogEntry[] = [];
+    for (const line of content.split("\n")) {
+      if (!line.trim()) continue;
+      try {
+        entries.push(JSON.parse(line) as AgentLogEntry);
+      } catch {
+        // skip malformed lines
+      }
+    }
+    return entries;
+  }
+
   getRootDir(): string {
     return this.rootDir;
   }

packages/core/src/types.ts

@@ -14,6 +14,18 @@ export interface TaskLogEntry {
   outcome?: string;
 }
 
+/** A single chunk of agent output (text delta or tool invocation) persisted to disk. */
+export interface AgentLogEntry {
+  /** ISO-8601 timestamp of when the entry was recorded */
+  timestamp: string;
+  /** The task this log entry belongs to */
+  taskId: string;
+  /** The text content (delta for "text", tool name for "tool") */
+  text: string;
+  /** Whether this is a text delta or a tool invocation marker */
+  type: "text" | "tool";
+}
+
 export interface TaskAttachment {
   filename: string;
   originalName: string;