feat: §17 ToolchainHandler 强约束实现（Step 1-4）

Step 1: 基础设施
- prompt_composer.py: PromptContext 新增 action_type + action_steps 字段
- spawner.py: handler 路径提取 action_type/action_steps 传入 PromptContext
- db.py: comments CHECK 约束加入 action_report

Step 2: ToolchainHandler 强化
- ToolchainContextSection: 加 steps 渲染 + action_hint（按 action_type）
- ToolchainApiSection: 改为 action_report 提交指引 + Gitea 协作指引
- ToolchainConstraintsSection: 5 条强约束 + Red Flags 防self-rationalization
- verify_completion: action_report → output → comment 三层 fallback
  - review_merged 始终通过（纯通知）
  - infrastructure_failure 始终通过（防递归）
  - 修复 LENGTH(content) → LENGTH(body) bug
- on_failure 三分路: 业务→Gitea PR comment / 系统→Gitea Issue / 基础设施→toolchain task

Step 3: toolchain_routes 改造
- 新增 _toolchain_db_path() + _send_toolchain_task()
- 所有 8 个 handler 改为 _send_toolchain_task
- _send_mail 保留但不再被 toolchain handler 调用
- _send_deploy_failure_mail → _send_deploy_failure_task

Step 4: 测试
- 29 个单元测试全部通过
- 全量 456 passed, 3 skipped, 0 failures

src/blackboard/db.py

@@ -293,7 +293,7 @@ _SCHEMA_STATEMENTS = [
         id INTEGER PRIMARY KEY AUTOINCREMENT,
         task_id TEXT NOT NULL REFERENCES tasks(id),
         author TEXT NOT NULL,
-        comment_type TEXT NOT NULL DEFAULT 'general' CHECK (comment_type IN ('general','handoff','observation','review','rebuttal','rebuttal_response','debate_argument','debate_rebuttal','debate_judgment')),
+        comment_type TEXT NOT NULL DEFAULT 'general' CHECK (comment_type IN ('general','handoff','observation','review','rebuttal','rebuttal_response','debate_argument','debate_rebuttal','debate_judgment','action_report')),
         body TEXT NOT NULL,
         mentions TEXT,
         created_at TEXT NOT NULL DEFAULT (datetime('now'))