docs(hooks): correct UserPromptSubmit prompt field type to ContentPart[]

[itxaiohanglover]

Problem

The Hooks documentation (docs/en/customization/hooks.md and docs/zh/customization/hooks.md) leads hook authors to believe the UserPromptSubmit payload field prompt is the submitted text string. In reality, the code sends the raw ContentPart[] array.

Source verification:

• packages/agent-core/src/agent/turn/index.ts — applyUserPromptHook triggers the hook with inputData: { prompt: input } where input is readonly ContentPart[].
• packages/agent-core/src/session/hooks/engine.ts — triggerInner flattens matcherValue to text via matcherValueText(...) for the matcher regex, but spreads ...args.inputData (including prompt) into the payload verbatim. The payload prompt field never goes through the text join.

A hook that reads payload.prompt expecting a string gets [{"type":"text","text":"hello"}] instead, so naive string handling (payload.prompt.includes(...), .toLowerCase(), regex, etc.) silently produces wrong/empty results.

What changed

• Event Reference table (UserPromptSubmit row): updated the description in both EN and ZH docs to mention that the payload includes a prompt field of type ContentPart[].
• Added a tip callout after the Event Reference table in both EN and ZH docs that:
  • States prompt is a ContentPart[] array, not a plain string.
  • Shows the actual payload shape: {"prompt": [{"type": "text", "text": "user's message"}]}.
  • Explains that the matcher regex is applied to the joined text of those parts (which is why the Event Reference says "The text submitted by the user" — that describes the matcher, not the payload field).
  • Includes a code snippet showing how to recover the plain text: payload.prompt.filter(p => p.type === 'text').map(p => p.text).join(' ').

Docs-only change, no source code or behavior changes.

Fixes #917

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