docs: drop process records and internal implementation docs

Keep only install, usage, development-workflow and architecture-boundary
documents; remove the one-off refactor plans, audits and ledgers that had
already served their purpose.

- delete docs/refactor plus the agent API surface audit generator and its
  drift-only tests; the fixed route registry remains the source of truth
- delete internal implementation notes (agent, music, notification, transfer,
  subscription activity, docker startup, V3t runtime governance)
- delete site adapter capture/release shims and the ADR records; external
  links point at MoviePilot-Site-Collector instead
- keep docs/architecture-overview.md and gate its dependency and event
  figures against the generated fixtures
- drop doc-only assertions from the architecture baseline test and retarget
  remaining references at code and rule documents

docs/testing.md

@@ -265,8 +265,6 @@ def test_recognize_prefers_explicit_identity(sample_meta, monkeypatch):
 - **没有直接 `assert` 不等于空测试**：`pytest.raises`、辅助断言、迁移 round-trip、导入兼容和“不抛异常”都是有效契约；只有名称或注释声称存在的覆盖则需要进一步核实。
 - **删除后验证**：运行受影响文件，核对保留用例及生产代码行/分支覆盖；跨领域清理再运行 `python tests/run.py`。不降低覆盖率基线、不新增 skip、不削弱架构或兼容门禁。
 
-首轮扫描范围、删除映射和保留理由见 [后端测试清理审计](testing-cleanup.md)。
-
 ## 排查测试问题
 
 - **收集报错（collection error）**：多为 import 期副作用或顶层桩污染。优先改成真实 import（conftest 已隔离临时库，真实 `settings`/helper 可加载）+ 方法内 patch，而不是靠事后还原（收集期污染发生在 import 那一刻，事后还原太晚）。