docs: resync AGENTS.md and the doc comments with the code

AGENTS.md:
- db.rs row claimed a per-store connection pool; there is one shared pool for
  all three tables (statics.rs builds it once).
- retry enqueue moved to send.rs, noted in both handlers rows.
- queue row now names both notifies (workers' + the sweep's).
- Retries bullet documents fetch vs fetch_once.
- test count ~125 -> ~135, the untested-files list no longer claims state.rs
  and handlers.rs are untested, and the live-test inventory mentions the
  token-gated, not-#[ignore]d pixiv download test that makes a local
  `cargo test --workspace` hit the network.
- /bot_dict is admin-only now.

Code docs:
- site/mod.rs: the module doc pointed new sites at `fetch_once` (a name that
  did not exist then and now means a single-attempt fetch) -> `SITES`; the
  cache_key/SITES/Site docs still said "twitter -> bsky -> pixiv" (misskey
  is registered third); RenderData now documents which fields are escaped
  and why url/author_url are not.
- state.rs, callback.rs: drop the pre-misskey site list and the `<name>`
  that rustdoc read as an HTML tag.
- Fixed the remaining rustdoc links/warnings: `cargo doc --workspace
  --no-deps` is now warning-free (was 8).
- docs/site-registry-refactor.md: §1 describes the pre-refactor state; said so.

No behavior change. fmt/clippy clean, 55 + 68 tests pass (the live pixiv
download test flaked on a CDN body timeout, as before).

docs/site-registry-refactor.md

@@ -10,6 +10,10 @@
 
 ## 1. 现状摩擦清单
 
+> ⚠️ 本节记录的是**重构前**的现状：其中的行号、以及 `site/mod.rs` 里的
+> `fetch_once` 分派函数（当时的实现）都已不存在，仅作历史记录。当前形态见
+> `site/mod.rs` 的 `SITES` 注册表——新增站点 = 新模块 + 注册一行。
+
 以现有三站（twitter / bsky / pixiv）为基线，新增第 4 个站点（代号 `example`）
 今天需要触碰的位置：