Implement LLM-WP-0008: provider-scoped account balance CLI

Add pluggable balance registry, OpenRouter credits/key limit client,
and llm-connect balance with one-shot --provider that does not change
library defaults.

llm_connect/_http.py

@@ -38,7 +38,36 @@ def post_json(
         headers={"Content-Type": "application/json", **(headers or {})},
         method="POST",
     )
+    return _read_json_response(url, req, timeout=timeout)
 
+
+def get_json(
+    url: str,
+    headers: Optional[Dict[str, str]] = None,
+    timeout: int = 60,
+) -> Dict[str, Any]:
+    """GET *url* and return the parsed JSON response body.
+
+    Raises:
+        LLMRateLimitError: on HTTP 429
+        LLMAPIError: on other non-2xx responses
+        LLMTimeoutError: on socket / read timeout
+    """
+    record_provider_request(url=url, payload=None, headers=headers or {})
+    req = urllib.request.Request(
+        url,
+        headers={**(headers or {})},
+        method="GET",
+    )
+    return _read_json_response(url, req, timeout=timeout)
+
+
+def _read_json_response(
+    url: str,
+    req: urllib.request.Request,
+    *,
+    timeout: int,
+) -> Dict[str, Any]:
     try:
         with urllib.request.urlopen(req, timeout=timeout) as resp:
             body = resp.read().decode()