A `Runtime` trait (run / compact / archive) with two backends:
- claude: a pass-through to hive_claude's InfiniteSession and
SessionStore, so a claude turn is the same spawn, session handling
and errors as before.
- acp: a generic Agent Client Protocol client. It spawns the command,
args and env from RuntimeSpec (HIVE_RUNTIME / HIVE_ACP_COMMAND /
HIVE_ACP_ARGS / HIVE_ACP_ENV), refuses an agent whose
mcpCapabilities.http is not true, passes the claude --mcp-config
servers as ACP mcpServers, keeps one session id in a file
(session/load after a restart, session/new otherwise), and maps
session/update into claude stream-json events plus usage_update into
Telemetry. Permission requests are answered by a caller-supplied
policy on the ACP tool kind. compact returns Unsupported for now.
The crate depends on no hyperhive binary crate, so the subagent
daemon can move onto it without pulling in hive-agent.
Refs #4391
