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

CLAUDE.md

@@ -114,8 +114,9 @@ hand-maintained per-file tree drifts out of sync with the code.
   directly over streamable-http (no stdio bridge), writes task files
   under `/harness/bash-tasks/`, and records the favorite-tools
   `bash_commands` stat into turn-stats.sqlite.
-- **`hive-subagent-mcp/`** — per-agent claude-subagent runner daemon
-  (`hive-subagent-daemon`); spawns nested claude sessions on request and
+- **`hive-subagent-mcp/`** — per-agent subagent runner daemon
+  (`hive-subagent-daemon`); spawns nested sessions on request, on the
+  parent agent's runtime via `hive-runtime` (claude or ACP), and
   serves `start`/`continue`/`status`/`interrupt` directly over
   streamable-http (no stdio bridge), plus a second subagent-facing route
   carrying `goal_reached`/`need_help` — served per session under a minted