feat(KB-335): rename environment variables from KB_* to FUSION_*

- Rename all KB_* environment variables to FUSION_* prefix across CLI and dashboard
- Update CLI runtime files (native-patch.ts) and build configuration
- Update dashboard server, badge-pubsub, GitHub webhooks, and terminal service
- Update test files to use new FUSION_API_KEY and related env vars
- Update documentation in README, AGENTS.md, STANDALONE.md, and dashboard README
- Update changeset files with new environment variable names

packages/cli/build.ts

@@ -230,7 +230,7 @@ function compileBinary(outFile: string, target: string, isCrossCompile: boolean)
       ...process.env,
       NODE_PATH: join(workspaceRoot, "node_modules"),
       // Tell the runtime where to find native assets
-      KB_RUNTIME_DIR: join(outDir, "runtime"),
+      FUSION_RUNTIME_DIR: join(outDir, "runtime"),
     },
   });