Document OpenCode prompt format options in ACP handler

[Copilot]

OpenCode accepts two prompt formats per ACP spec. Our codebase uses prompt array format while the PoC used content array format, causing potential confusion.

Changes

Added inline documentation to sendPrompt method clarifying both valid formats:

private async sendPrompt(
  process: ChildProcess,
  sessionId: string,
  content: string
): Promise<unknown> {
  // NOTE: OpenCode supports multiple prompt formats:
  // - prompt: [{ type: 'text', text: '...' }] (our format)
  // - content: [{ role: 'user', content: '...' }] (alternative)
  // Both are valid per ACP spec.
  return this.sendRequest(
    process,
    'session/prompt',
    {
      sessionId,
      prompt: [
        {
          type: 'text',
          text: content,
        },
      ],
    },
    60
  );
}

This comment documents our implementation choice and prevents confusion when comparing against other ACP implementations or the PoC.

Original prompt

Add a code comment explaining that OpenCode supports multiple prompt formats and clarify the team's choice of format. Specifically, the Proof of Concept (PoC) uses the 'content' array format whereas the current implementation uses the 'prompt' array format.

Detailed reference:

The PoC uses:

send('session/prompt', {
  sessionId: sessionId,
  content: [  // PoC uses 'content' array
    {
      role: 'user',
      content: 'Write a hello world program in Python.'
    }
  ]
});

The current implementation uses:

{
  sessionId,
  prompt: [  // We use 'prompt' array
    {
      type: 'text',
      text: content,
    }
  ],
}

Proposed solution:

Include the following comment in acp-handler.ts at the relevant method in the range src/core/target-handlers/acp-handler.ts:198-217:

private async sendPrompt(
  process: ChildProcess,
  sessionId: string,
  content: string
): Promise<unknown> {
  // NOTE: OpenCode supports multiple prompt formats:
  // - prompt: [{ type: 'text', text: '...' }] (our format)
  // - content: [{ role: 'user', content: '...' }] (alternative)
  // Both are valid per ACP spec.
  return this.sendRequest(
    process,
    'session/prompt',
    {
      sessionId,
      prompt: [
        {
          type: 'text',
          text: content,
        },
      ],
    },
    60
  );
}

Please implement this change and address Issue #999.

This pull request was created from Copilot chat.

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