hive-runtime: shared runtime crate with claude and acp backends

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

Cargo.toml

@@ -19,6 +19,7 @@ members = [
   "hive-metric",
   "hive-priv",
   "hive-priv-sock",
+  "hive-runtime",
   "hive-sh4re",
   "hive-sock-client",
   "hive-subagent-mcp",
@@ -99,6 +100,7 @@ hive-core-agent-sock = { path = "hive-core-agent-sock" }
 hive-claude = "0.1.1"
 hive-host-sock = { path = "hive-host-sock" }
 hive-priv-sock = { path = "hive-priv-sock" }
+hive-runtime = { path = "hive-runtime" }
 hive-sock-client = { path = "hive-sock-client" }
 hive-types = { path = "hive-types" }
 swarm-authelia-bridge-sock = { path = "swarm-authelia-bridge-sock" }