subagent: give a run a goal, turns toward it, and a reason it stopped

`start` takes an optional `goal`. With one set a session stops being a
single turn: when a turn ends and nothing has said to stop, the daemon
spawns another turn re-prompting the subagent toward that goal, up to
`max_turns` (default 5, per-session). Without a goal nothing changes —
one turn, one todo, same as before.

Four things end a run, each recorded distinctly and reported by `status`:
the turn ending with no goal, `goal_reached`, `need_help`, and the turn
cap. The last says so out loud rather than stopping quietly — the todo
states the harness limit was reached and the goal was never reported
reached. Every stop extends the done message rather than replacing it,
and lands in the session's report file when it has one. The path is
never inferred: it comes from `start`'s `report_file` or from the
subagent naming where it wrote.

`goal_reached` and `need_help` are the subagent's own, served on a second
route (`/signal/mcp`) that carries those two tools and nothing else, so
reporting on a run can't become starting one. `goal_reached` is built as
a label, never a gate: it is self-reported by a subagent that has just
been re-prompted with "you haven't reached the goal", which is exactly
the incentive to claim it — the same failure class as a build report
asserting the tests pass. Every surface that renders it says so.
`need_help` is the blocking signal, and shows in `status` as its own
state so a parent polling it sees the block without reading a file.

`status` also carries `turn N of M`: with 4330's last-event age, that
separates working from wedged from out of turns off one answer.

Two bugs the new tests caught: a `tokio::fs::File` was dropped without
flushing, so the report line was written to nothing, and the plain idle
answer dropped the turn counter.

Also documents `await_resume`'s third case — a closed channel with no
send, which fails open the same as `Underway` — per argus on #4411.

Refs #4403

hive-subagent-mcp/src/main.rs

@@ -1,7 +1,9 @@
 //! `hive-subagent-daemon` binary — spawns nested claude sessions on
-//! request and serves the `start`/`continue`/`interrupt` MCP tool surface
-//! directly over streamable-http on `--http <addr>` — no stdio bridge, no
-//! separate bin claude has to respawn every turn.
+//! request and serves the `start`/`continue`/`status`/`interrupt` MCP tool
+//! surface directly over streamable-http on `--http <addr>` — no stdio
+//! bridge, no separate bin claude has to respawn every turn. The same
+//! address also carries the subagent-facing `goal_reached`/`need_help`
+//! route, which is the only place the daemon can learn its own URL from.
 
 use std::sync::Arc;
 
@@ -42,7 +44,14 @@ async fn main() -> Result<()> {
         "hive-subagent-daemon starting"
     );
 
-    let state = Arc::new(hive_subagent_mcp::session::State::new(todo_socket));
+    // The one place the subagent-facing signal URL can come from: the
+    // address this process was told to listen on. Anything else would be a
+    // guess at the deployment's own port assignment.
+    let signal_url = format!("http://{}{}", cli.http, hive_subagent_mcp::mcp::SIGNAL_PATH);
+    let state = Arc::new(hive_subagent_mcp::session::State::new(
+        todo_socket,
+        signal_url,
+    ));
 
     // Serve the MCP tools over streamable-http forever. No background poll
     // loop to start — unlike the bash daemon's task-file queue, `start`/