ci: raise coverage thresholds from 0% to meaningful levels

[zoeshawwang]

加两道 CI 卡点，把 SDK 的回归保护从"形同虚设"提到"防得住典型回归"。

开发者改了 __xxx_async_template.py 但忘跑 make codegen，对应的 xxx.py 就和 template 脱节，下次别人跑 codegen 会被覆盖。CI 现在
跑 make codegen && git diff --exit-code，发现 diff 直接 fail 并提示 本地补跑 make codegen 后提交。

零风险：当前 main 上 make codegen 输出零 diff，新 step 立即就能过。

coverage.yaml 之前 full / incremental 全设 0，等于不卡。
基于本地实测当前覆盖率（line 90.74% / branch 82.85%）设：

• 全量阈值：line 85 / branch 78（留 ~5% buffer 防 CI 抖动误伤）
• 增量阈值：line 85 / branch 75（推动新代码自带测试）
• directory_overrides：
  • utils: 90/90（基础设施代码，保持高线）
  • knowledgebase: 95/90（历史高覆盖）
  • memory_collection: 80/50（历史短板，不严于现状，先不回退）
  • conversation_service: 65/60（历史短板，同上）

uv run --python 3.10 --all-extras python scripts/check_coverage.py 本地跑通 23 项检查全 ✅。

• pyink/isort 格式 check：当前 19 个存量文件不合规（含 tests/e2e/test_workspace_id.py 等），需要先 reformat 再加 check。
• ruff/pylint lint check
• 多 Python 版本矩阵（目前只跑 3.10）
• E2E on CI（需要 Secret 配 AccessKey）

Change-Id: I8bd1fb2f3262fe77143d606e5f0ebb9125cf0932
Co-developed-by: Claude noreply@anthropic.com

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