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.

Cargo.lock

@@ -1521,7 +1521,6 @@ dependencies = [
  "anyhow",
  "axum",
  "clap",
- "forgejo-api",
  "futures-util",
  "hive-agent-sock",
  "hive-claude",
@@ -1538,13 +1537,11 @@ dependencies = [
  "serde",
  "serde_json",
  "tempfile",
- "time",
  "tokio",
  "tokio-stream",
  "tower-http 0.7.0",
  "tracing",
  "tracing-subscriber",
- "url",
 ]
 
 [[package]]
@@ -1671,6 +1668,23 @@ dependencies = [
  "url",
 ]
 
+[[package]]
+name = "hive-forge-notify"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "forgejo-api",
+ "hive-agent-sock",
+ "reqwest 0.13.1",
+ "serde",
+ "serde_json",
+ "time",
+ "tokio",
+ "tracing",
+ "tracing-subscriber",
+ "url",
+]
+
 [[package]]
 name = "hive-host-sock"
 version = "0.1.0"