feat(react): add useChat hook package

Author: marslavish

packages/react/README.md

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+## @agentic-kit/react
+
+Headless React bindings for `agentic-kit`.
+
+Currently exposes a single hook:
+
+- `useChat({ api, body, initialMessages, onMessage, onFinish, onDecisionPending })`
+  POSTs `messages` to `api`, parses the SSE response into `AgentEvent`s, and
+  folds them into a `Message[]`. Surfaces pause/resume via `pendingDecision` +
+  `respondWithDecision(toolCallId, value)`.
+
+The hook ships no UI. State lives in messages — there is no separate run
+store, no `runId`. Resumption after a tool decision re-POSTs to the same `api`
+endpoint with the augmented message log.

packages/react/__tests__/tsconfig.json

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+{
+  "extends": "../tsconfig.json",
+  "compilerOptions": {
+    "noEmit": true,
+    "rootDir": "../../..",
+    "baseUrl": "../../..",
+    "paths": {
+      "@test/*": ["tools/test/*"],
+      "agentic-kit": ["packages/agentic-kit/src"],
+      "@agentic-kit/agent": ["packages/agent/src"],
+      "@agentic-kit/react": ["packages/react/src"]
+    },
+    "types": ["jest", "node"]
+  },
+  "include": [
+    "./**/*.ts",
+    "./**/*.tsx",
+    "../src/**/*.ts",
+    "../src/**/*.tsx",
+    "../../../tools/test/**/*.ts"
+  ],
+  "exclude": ["../dist", "../node_modules"]
+}