subagent: give each run its own signal URL, and drop the name argument

`goal_reached`/`need_help` took the session name as a tool argument, so
identity was an assertion by the caller and the only guard on it was
`occupancy()` — "does that name have a turn in flight", which two
concurrently running siblings both satisfy for each other. A subagent
could stop its sibling's run by naming it.

Identity moves into the URL. Each spawned run is minted an unguessable
token (`Uuid::new_v4`, the OS CSPRNG), the URL carrying it goes into that
one subagent's own `--mcp-config`, and the route resolves it back to a
session before dispatching to a handler bound to that session. Neither
tool takes a `name` any more: a subagent has no field in which to name a
sibling, and a sibling's name — which a brief may well mention — is not a
token.

One route with a path parameter, not a route per session: the `Router` is
built once at startup and subagents come and go for the daemon's whole
life. An unminted or revoked token gets a bare 404, the same answer either
way, so nothing enumerates. A run's token is revoked when the run ends
(`finish_turn`) or when a call never reached a spawn.

Two things fall out of that:

- the config file becomes one per session. A single shared path was
  already a race between two `start`s; with a per-session URL in it, the
  loser would read the winner's identity.
- `occupancy()` stops being the identity guard and is gone from the signal
  path entirely rather than kept "just in case" — a revoked token can't
  reach it, and it never answered the question it was standing in for.
  It still backs `status`, which is what it was always actually for.

Refs #4403
Refs #4413

hive-subagent-mcp/src/main.rs

@@ -44,13 +44,16 @@ async fn main() -> Result<()> {
         "hive-subagent-daemon starting"
     );
 
-    // The one place the subagent-facing signal URL can come from: the
+    // The one place the subagent-facing signal URLs 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);
+    // guess at the deployment's own port assignment. This is only their
+    // common prefix — each session's actual URL is this plus a token minted
+    // for that session alone, which is what makes a signal's identity a
+    // property of the endpoint rather than of the payload.
+    let signal_base = format!("http://{}{}", cli.http, hive_subagent_mcp::mcp::SIGNAL_PATH);
     let state = Arc::new(hive_subagent_mcp::session::State::new(
         todo_socket,
-        signal_url,
+        signal_base,
     ));
 
     // Serve the MCP tools over streamable-http forever. No background poll