wip(core): v2 subagent foundations (awaitIdle, wait, create parentID)

Checkpoint of the safe subagent slice:
- SessionExecution.awaitIdle (coordinator drain-to-quiescence loop)
- SessionV2.wait delegates to awaitIdle; drop unavailable error + handler catch
- SessionV2.create: parentID/title, optional location inherited from parent
- subagent tool file (core/src/tool/subagent.ts) NOT yet wired into the node
  graph; wiring the global SessionV2 dep from a location-scoped tool is the open
  problem being experimented on next.

packages/server/src/handlers/session.ts

@@ -222,14 +222,6 @@ export const SessionHandler = HttpApiBuilder.group(Api, "server.session", (handl
                 }),
               ),
             ),
-            Effect.catchTag("Session.OperationUnavailableError", (error) =>
-              Effect.fail(
-                new ServiceUnavailableError({
-                  message: `Session ${error.operation} is not available yet`,
-                  service: `session.${error.operation}`,
-                }),
-              ),
-            ),
           )
           return HttpApiSchema.NoContent.make()
         }),