docs(agents): type the React example's root id as ConversationId

[eersnington]

• examples/docs/react-sdk/useAgentChat.ts stores the root conversation id as ConversationId, so submit and abort type-check now that the actions take Pi's branded ids.

-const [rootId, setRootId] = useState<number | null>(null);
+const [rootId, setRootId] = useState<ConversationId | null>(null);

• The file is outside the examples type check until RivetKit types client event payloads, so CI does not catch this.

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This is part 9 of 9 in a stack:

•  9  docs(agents): type the React example's root id as ConversationId #346 👈
•  8  feat(pi): prefix Pi Durable's tables in actor sqlite #345
•  7  chore(pi): bump and test Pi Durable 1.1.0 #344
•  6  refactor(pi): type action ids with Pi's branded id types #343
•  5  refactor(sandbox-adapter): simplify sandboxEnv #342
•  4  refactor(pi): move databaseEnv into pi and version pi_file #341
•  3  fix(sandbox-adapter): stop waiting on aborted Daytona exec #340
•  2  fix(sandbox-adapter): stop Pi env calls when the run aborts #339
•  1  fix(pi): read an app credential with one source call #338