Initial aether repository structure

Distributed actor system with event sourcing for Go:
- event.go - Event, ActorSnapshot, EventStore interface
- eventbus.go - EventBus, EventBroadcaster for pub/sub
- nats_eventbus.go - NATS-backed cross-node event broadcasting
- store/ - InMemoryEventStore (testing), JetStreamEventStore (production)
- cluster/ - Node discovery, leader election, shard distribution
- model/ - EventStorming model types

Extracted from arcadia as open-source infrastructure component.

Co-Authored-By: Claude <noreply@anthropic.com>

.gitea/workflows/ci.yaml

@@ -0,0 +1,19 @@
+name: CI
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-go@v5
+        with:
+          go-version: '1.23'
+      - name: Build
+        run: go build ./...
+      - name: Test
+        run: go test ./...