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).

crates/xmedia-bot/src/handlers/callback.rs

@@ -1,5 +1,5 @@
-//! Callback query handling: the edit-before-forward prompt's "forward" and
-//! "template|<name>" buttons.
+//! Callback query handling: the edit-before-forward prompt's `"forward"` and
+//! `"template|<name>"` buttons.
 
 use super::{CHAT_STORE, CONFIG, TASK_QUEUE};
 use crate::db::unix_now;