docs(websocket): mark sharpapi-v1.5 as internal-only (#204)

The MessagePack subprotocol is reserved for first-party SDKs and SharpLines
— it isn't part of the documented public API. Listing it in the public
asyncapi spec led a customer-support agent to recommend it to a customer
debugging unrelated 60s WS disconnects (already fixed server-side in
sharp-api-go PR #177).

Removes v1.5 from the public table and replaces the explanatory paragraph
with a note that additional subprotocols exist but are not public.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Mlaz-code

public/asyncapi.yaml

@@ -12,18 +12,18 @@ info:
 
     ## Subprotocols
 
-    The server negotiates one of two subprotocols on upgrade. Pass the chosen
-    name in the `Sec-WebSocket-Protocol` header (browsers: second arg to the
-    `WebSocket` constructor).
-
-    | Subprotocol      | Wire format for `data` payloads          | When to use                       |
-    |------------------|------------------------------------------|-----------------------------------|
-    | `sharpapi-v1`    | JSON throughout                          | Browsers, simple clients          |
-    | `sharpapi-v1.5`  | JSON control frames + MessagePack data   | High-throughput servers, SDKs     |
-
-    Both protocols deliver the same logical messages with the same field names;
-    `sharpapi-v1.5` swaps the `data` payload binary encoding for ~30% smaller
-    frames at the cost of needing a MessagePack decoder.
+    Public clients negotiate `sharpapi-v1` by passing it in the
+    `Sec-WebSocket-Protocol` header (browsers: second arg to the `WebSocket`
+    constructor). It carries JSON throughout.
+
+    | Subprotocol   | Wire format for `data` payloads | When to use         |
+    |---------------|---------------------------------|---------------------|
+    | `sharpapi-v1` | JSON throughout                 | All public clients  |
+
+    The server may also accept additional subprotocols reserved for first-party
+    SDKs and internal services. Those are not part of the documented public API
+    and may change without notice — public clients should always negotiate
+    `sharpapi-v1`.
 
     ## Authentication