refactor(hive-agent): split the forge notification poller into its own crate

The poller was a `tokio::spawn` inside the `hive-agent` serve loop. It
never needed anything from that loop except a socket path, so being
in-process bought nothing and cost two things: a harness restart took
forge notifications down with it, and the whole forge/HTTP dependency
tree was linked into the serve-loop binary.

It is now `hive-forge-notify`, a per-agent daemon with its own systemd
unit, a sibling of `hive-bash-daemon` and `hive-matrix-daemon`. Same
contract as those two: it reaches the harness only by upserting todos on
the in-agent socket, and nowhere else.

The module moves verbatim (`notify.rs`) — the formatters, the activation
gates, the dedupe map and all 33 tests are unchanged. Only the socket
call sites are rewritten, onto a small local `todo_client` rather than
the harness's. That mirrors what both sibling daemons already do, and
the etiquette differs on purpose: the harness's client carries a 60s
backoff schedule sized to ride out a hive-c0re restart, which its
callers need because they have no retry of their own. This poller's two
call sites both sit inside the 30s poll loop and both treat a failure as
"leave the thread unread, try next tick", so the poll interval already
is the retry; a second backoff would only stack sleeps and delay the
rest of the batch.

The unit is `Restart=on-failure`, not `always`. An agent with no forge
account is a supported configuration and the poller reports it by
logging why and exiting 0 — under `always` that clean exit would be a
restart loop on every forge-less agent.

`forgejo-api`, `url` and `time` drop out of `hive-agent`'s dependencies
with the module.

Also corrects docs that outlived the code they described: the persisted
`forge_cursor` field is long gone (forge's own read-state is the durable
record of what has been delivered), but `docs/persistence.md` and the
`harness_state` module docs still documented it as live.

docs/turn-loop.md

@@ -97,9 +97,11 @@ flake sets it unconditionally for any container-deployed agent;
 see `docs/conventions.md::Hive identity` for the env stack),
 opens turn-stats sqlite, prepares the on-boot files (see
 [claude-invocation](turn-loop/claude-invocation.md#on-boot-files)),
-installs claude plugins, spawns `forge_notify::run` + `web_ui::serve`,
+installs claude plugins, spawns `web_ui::serve` + `vacuum::run`,
 and either drops into `serve_loop` directly (`Online`) or parks on
-the login flow first (`NeedsLogin`).
+the login flow first (`NeedsLogin`). (The forge notification poller
+used to be spawned here too; it is its own process now —
+`hive-forge-notify`, see [`forge.md`](forge.md).)
 
 `spawn_todo_socket` opens the todos store and, alongside
 `todo_server::run` (the socket the out-of-process producers dial),