fix: update blockchain block command to use local node RPC endpoints with fallback search, revert client job submission to /v1/jobs, and document Concrete ML Python 3.13 compatibility limitation

- Change blockchain block command to query local node at /rpc/blocks/by_hash/{hash} instead of coordinator
- Add fallback to /rpc/blocks/{height} for numeric block identifiers
- Implement block hash search across recent blocks (last 10) when direct lookup fails
- Revert client job submission endpoint from

cli/aitbc_cli/commands/client.py

@@ -48,7 +48,7 @@ def submit(ctx, job_type: str, prompt: Optional[str], model: Optional[str],
         try:
             with httpx.Client() as client:
                 response = client.post(
-                    f"{config.coordinator_url}/api/v1/jobs",
+                    f"{config.coordinator_url}/v1/jobs",
                     headers={
                         "Content-Type": "application/json",
                         "X-Api-Key": config.api_key or ""