feat(core,server,web,cli): mid-run steering messages and square-bracket markers (#63)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

packages/core/package.json

@@ -18,6 +18,10 @@
       "types": "./dist/omnimessage/index.d.ts",
       "import": "./dist/omnimessage/index.js"
     },
+    "./markers": {
+      "types": "./dist/omnimessage/markers/index.d.ts",
+      "import": "./dist/omnimessage/markers/index.js"
+    },
     "./interfaces": {
       "types": "./dist/interfaces.d.ts",
       "import": "./dist/interfaces.js"