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

nix/agent-modules/agent-service.nix

@@ -228,6 +228,10 @@ in
       On `"acp"`, `model`, `effortLevel` and `autoCompact` have no effect
       (the model is whatever the agent is configured with), and neither the
       web UI's cancel button nor `/compact` works yet.
+
+      Subagents (`hive-subagent-daemon`) run on the same runtime. On
+      `"acp"`, each subagent run spawns its own copy of the ACP agent, which
+      also gets `services.hyperhive.agent.backendEnvironmentFile`.
     '';
   };