fix(lint): 修复 PR #14 引入的 lint 回退 (119→0)

PR #14 从旧分支复制文件导致回退了 PR #10 的 lint 修复。
修复内容:
- autoflake 移除未使用导入/变量
- autopep8 修复缩进/空格
- 手动修复 F821(pathlib→Path), F541(f-string), F841(未使用变量)
- 所有修复均通过 flake8 --max-line-length=120 --extend-ignore=E501 检查 (0 errors)

src/blackboard/db.py

@@ -4,7 +4,6 @@ from __future__ import annotations
 
 import sqlite3
 from pathlib import Path
-from typing import Optional
 
 
 def init_db(db_path: Path) -> None:
@@ -133,8 +132,10 @@ def _migrate_v28(conn: sqlite3.Connection) -> None:
         resolved_by TEXT,
         resolve_note TEXT
     )""")
-    conn.execute("CREATE INDEX IF NOT EXISTS idx_checkpoints_task ON checkpoints(task_id)")
-    conn.execute("CREATE INDEX IF NOT EXISTS idx_checkpoints_status ON checkpoints(status)")
+    conn.execute(
+        "CREATE INDEX IF NOT EXISTS idx_checkpoints_task ON checkpoints(task_id)")
+    conn.execute(
+        "CREATE INDEX IF NOT EXISTS idx_checkpoints_status ON checkpoints(status)")
 
     # 4. outputs 扩展字段（M3 成果物）
     _safe_add_column(conn, "outputs", "file_name", "TEXT")
@@ -189,18 +190,20 @@ TERMINAL_STATUSES = frozenset()  # v3.1: 无终态，全靠 VALID_TRANSITIONS
 MANUAL_STATUSES = frozenset({"cancelled", "paused", "reviewing"})
 
 VALID_TRANSITIONS = {
-    "pending":       {"claimed", "paused", "blocked", "cancelled"},
-    "claimed":       {"working", "paused", "pending", "cancelled"},
-    "working":       {"review", "done", "blocked", "failed", "paused", "escalated", "waiting_human", "cancelled", "pending"},  # pending: Mail spawn 失败回退
-    "paused":        {"working", "claimed", "review", "escalated", "waiting_human", "cancelled"},  # 恢复到 resumed_from 记录的状态
-    "review":        {"done", "pending", "failed", "paused", "escalated", "waiting_human", "cancelled"},
-    "blocked":       {"pending", "escalated", "cancelled"},
-    "failed":        {"pending", "escalated", "cancelled"},
-    "escalated":     {"working", "pending", "paused", "cancelled"},
+    "pending": {"claimed", "paused", "blocked", "cancelled"},
+    "claimed": {"working", "paused", "pending", "cancelled"},
+    # pending: Mail spawn 失败回退
+    "working": {"review", "done", "blocked", "failed", "paused", "escalated", "waiting_human", "cancelled", "pending"},
+    # 恢复到 resumed_from 记录的状态
+    "paused": {"working", "claimed", "review", "escalated", "waiting_human", "cancelled"},
+    "review": {"done", "pending", "failed", "paused", "escalated", "waiting_human", "cancelled"},
+    "blocked": {"pending", "escalated", "cancelled"},
+    "failed": {"pending", "escalated", "cancelled"},
+    "escalated": {"working", "pending", "paused", "cancelled"},
     "waiting_human": {"working", "done", "paused", "cancelled"},
-    "done":          {"cancelled", "reviewing"},
-    "reviewing":     {"done", "working", "cancelled"},
-    "cancelled":     {"pending"},
+    "done": {"cancelled", "reviewing"},
+    "reviewing": {"done", "working", "cancelled"},
+    "cancelled": {"pending"},
 }
 
 COMMENT_TYPES = frozenset({
@@ -224,7 +227,8 @@ EVENT_TYPES = frozenset({
 
 OUTPUT_TYPES = frozenset({"code", "document", "data", "config", "other"})
 
-REVIEW_TYPES = frozenset({"plan_review", "output_review", "guardrail", "final_review"})
+REVIEW_TYPES = frozenset(
+    {"plan_review", "output_review", "guardrail", "final_review"})
 VERDICT_TYPES = frozenset({"approved", "rejected", "needs_revision"})
 
 EXPERIENCE_SOURCES = frozenset({