hive-subagent-mcp: run an agent's subagents on its runtime

The subagent daemon now reads the parent agent's runtime at startup
(`hive_runtime::RuntimeSpec`, from the harness's `HIVE_RUNTIME` /
`HIVE_ACP_*`, which `mcp.nix` forwards onto its unit). On claude
nothing changes. On ACP, each run drives an `AcpRuntime` whose session
id is kept per name under the harness dir: `start` archives the old one,
`continue` loads it (and fails when none is recorded), `interrupt` sends
`session/cancel`, a role goes in front of the first prompt, and
permission requests get the answers a claude subagent's tool list
gives. The unit loads `backendEnvironmentFile` on ACP only, so the
agent can authenticate.

The end-of-turn handling moves out of the claude loop into `after_turn`
unchanged, so both loops share it.

Refs #4391

hive-subagent-mcp/src/main.rs

@@ -51,10 +51,11 @@ async fn main() -> Result<()> {
     // 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_base,
-    ));
+    // The parent agent's runtime, from the same variables the harness reads.
+    let runtime = hive_runtime::RuntimeSpec::from_env()?;
+    let state = Arc::new(
+        hive_subagent_mcp::session::State::new(todo_socket, signal_base).on_runtime(runtime),
+    );
 
     // Serve the MCP tools over streamable-http forever. No background poll
     // loop to start — unlike the bash daemon's task-file queue, `start`/