feat: let the retention script be told which table it may delete from

Until now the target came from whatever search_path resolved to. The
script printed what it found, but that print and the DELETE happen in
the same run with nobody in between, so it only ever helped the person
who ran a dry-run first. Swap the role that runs it and "$user" can
resolve somewhere else entirely.

--table takes the whole qualified name and resolves it directly. The
table half has to be llm_calls: a version that accepts any name turns
one typo into a general purpose row deleter, and any table with a
created_at and a tenant_id would go through the same batched DELETE
without complaint.

The tests that run it now run as a role that owns its own scratch table
and holds nothing on the shared one, so the row-count snapshot could
go. What replaced it is a case that lets the script fall through to the
shared table on purpose and asserts it exits 2 having deleted nothing.
That one has no red-first path, since making it red means running it as
the superuser, which is the thing being prevented; the finding's probe
covers it instead.

Five of the new usage tests passed before the flag existed, because
argparse rejects an unknown --table with exit 1 and the word --table in
stderr, which is exactly what they asserted. They now also assert the
error is not "unrecognized", which is the difference between testing
the validation and testing argparse.

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

@@ -60,7 +60,7 @@ L3 不是测试的问题，是脚本契约的问题——它同时是生产风
 | 仅 `--backend postgres` 接受 | sqlite 给了 `--table` → 退出 **1** | 与 `--batch-size` 同款；SQLite 库文件即目标，无 schema 概念，无歧义可消 |
 | 必须是**两段**限定名 | `--table llm_calls` → 退出 **1**，提示写成 `schema.表名` | 单段等于没声明，隐式性原样保留 |
 | **表名段必须逐字等于 `llm_calls`** | `--table audit.events` → 退出 **1**，消息点明本脚本只清理 `llm_calls` | 见 §4.4：不加这条，`--table` 会把本脚本从"遥测表清理器"扩成"任意同形表删除工具" |
-| 两段均非空、均不含 `.` 与 `"` | 不合法 → 退出 **1** | 复杂标识符（含点/引号的表名）不支持，此时退回不给 `--table` 的路径；写进 `--help` |
+| 两段均非空、均不含 `"` | 不合法 → 退出 **1** | 复杂标识符（含引号的表名）不支持，此时退回不给 `--table` 的路径；写进 `--help`。**本行原写作"均不含 `.` 与 `\"`"，实现阶段核出"段内含 `.`"是不可达分支**——按 `.` 切分后恰好两段是前置条件，`a.b.c` 走的是"不是恰好两段"那条消息，故删去该半句 |
 | **逐字比较，不做大小写折叠** | 传 `_quote()` 包裹的限定名给 `to_regclass` | catalog 里存的是真实标识符；未加引号建的表在 catalog 中是小写。折叠会与"引号标识符区分大小写"的真实语义打架 |
 | 解析不到 | 退出 **2**，消息点名"显式指定的表 X 不存在"，并附一句"PG 中未加引号建的标识符在 catalog 里是小写" | 与 `search_path` 找不到的消息**分开写**：诊断方向不同。**退出码维持 2 而非 1**：`Public.llm_calls` 格式合法，找不到是环境事实而非参数非法——把它归成 1 会让"schema 真的不存在"这类该告警的情形被调度器当成不必重试的参数错误。大小写这类高频手误由消息文本消化，不由退出码 |
 | 无权限 | 后续 `COUNT` 抛 `PostgresError` → 既有 except → 退出 **2** | 无需新增分支 |