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.

CLAUDE.md

@@ -45,7 +45,7 @@ hand-maintained per-file tree drifts out of sync with the code.
 - **`hive-agent/`** — the serve-loop binary: turn-loop *policy* layer
   (`turn.rs`) over the `hive-claude` driver, per-agent web UI (`web_ui/`
   module dir), event + turn-stats sqlite sinks, login flow, system-prompt
-  renderer, forge-notify subscriber.
+  renderer.
 - **`hive-agent-mcp/`** — the embedded MCP server (long-lived
   streamable-http listener, `hive-mcp-http` systemd unit) + its claude
   launch-config layer (tool-group/capability → `--allowedTools`,
@@ -62,6 +62,10 @@ hand-maintained per-file tree drifts out of sync with the code.
   `docs/boundary.md`.
 - **`hive-forge/`** — `hive-forge` Forgejo CLI wrapper; one module per
   verb under `src/verbs/`.
+- **`hive-forge-notify/`** — per-agent Forgejo notification poller daemon
+  (`hive-forge-notify`); turns unread notification threads into todos on
+  the harness's in-agent socket. Was a task inside the `hive-agent` serve
+  loop; own process since it needs nothing else from the harness.
 - **`hive-matrix-mcp/`** — per-agent matrix-sdk daemon
   (`hive-matrix-daemon`); serves its MCP tools (`send_message`,
   `read_room`, …) directly over streamable-http (no stdio bridge),