CI: stand up Gitea Actions (TST-03) + fix the defects it caught (#123)

Make the authored CI pipeline execute and go green on the co-located runner (portable + java), fix the five real latent defects the first real run surfaced (codegen-check null, stale rust proto, orphan-terminator Linux path, stale java worker codegen, py3.12 event loop), provision pwsh + Gradle for the self-hosted act image, and disable the Windows jobs (act host-mode broken). TST-03 -> Done.

https://claude.ai/code/session_01DMXXvNuPekkkrTEyPNxEkW

clients/rust/protos/mxaccess_worker.proto

@@ -42,6 +42,12 @@ message GatewayHello {
   uint32 supported_protocol_version = 1;
   string nonce = 2;
   string gateway_version = 3;
+  // Maximum worker-frame payload size, in bytes, negotiated by the gateway from its
+  // configured pipe limit. The worker adopts this as its frame-protocol MaxMessageBytes
+  // instead of a hard-coded default; 0 (an older gateway that never set the field) means
+  // "use the worker's built-in default". Sits above the public gRPC cap by an
+  // envelope-overhead margin so an accepted gRPC payload always fits one worker frame.
+  uint32 max_frame_bytes = 4;
 }
 
 message WorkerHello {