Allow concurrency 0 to pause a terminal

A paused terminal gets no new runs and is not probed; in-flight runs still finish.

config/schema.json

@@ -53,7 +53,7 @@
         "repo": {"type": "string"},
         "plane_user": {"type": "string"},
         "token_key": {"type": "string"},
-        "concurrency": {"type": "integer", "minimum": 1},
+        "concurrency": {"type": "integer", "minimum": 0},
         "capabilities": {"$ref": "#/definitions/strings"},
         "executors": {"type": "array", "items": {"type": "string"}, "minItems": 1},
         "bins": {"type": "object", "additionalProperties": {"type": "string"}},