docs: order the issue #18 work so nothing deletes the shared table

The plan's one irreversible risk is the worst-case test itself. It
deliberately lets the script fall through to the shared table, and the
account in .env is a superuser, so running it before the sandbox role
exists would delete every expired row in there. All eleven rows on that
table predate any cutoff the tests use.

That forces the order: factory, then the retention tests move onto an
owner role, and only then does the case get written. Review caught that
the original order also made the --table integration cases impossible
to fail first, since the tool would already be implemented by the time
they were written. Same fix resolves both.

The worst-case case has no red-first path at all. Turning it red means
running it as the superuser, which is the thing being prevented, so its
evidence is the probe in the finding instead, and the plan says so
rather than calling it verified.

One acceptance criterion in the design turned out to be unrunnable: the
hint line only prints on the Postgres branch, so no unit test that
never connects can assert it. Corrected in place.

research-wiki/designs/2026-08-26-issue18-pg-test-isolation-design.md

@@ -206,7 +206,7 @@ tests/ 下不得出现字面量 public.llm_calls —— 命中即 exit 1
 |---|---|---|
 | 1a | `--table` 的**参数分类**：sqlite 互斥、非两段、空段、含点/引号、表名段非 `llm_calls` —— 各自退出 1 | 单测（`tests/unit/test_retention_tool.py`，无需 PG） |
 | 1b | `--table` 的**真实解析行为**：显式指向 sandbox 表成功删除；指向不存在的 schema → 2；指向无权表 → 2；指向分区表 → 仍 3 | **集成用例（必须真连 PG）**——单测只能验参数分类与拼出的目标字符串，验不了 `to_regclass` 的真实语义 |
-| 2 | 未给 `--table` 且 `--apply` 时打印推断提示 | 单测断言 stdout |
+| 2 | 未给 `--table` 且 `--apply` 时打印推断提示 | **集成用例**断言 stdout —— 该提示行只在 PG 分支打印，不连库的单测触发不到它（本行原写作"单测断言 stdout"，计划阶段核出该判据不可执行，就地更正） |
 | 3 | 最坏情况（search_path 落到 public）**删不掉任何行**且退出 2 | §5.3 新用例 |
 | 4 | 整套 `tests/integration` 连跑三次全绿，其间 `public.llm_calls` 行数由外部任意变动 | 连跑 + 期间手工改动共享表行数 |
 | 5 | `tests/` 下 `public.llm_calls` 零命中 | `make lint` |