[FEATURE]: ACP sessions should get auto-generated titles instead of "ACP Session <UUID>" #13139

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Describe the enhancement you want to request

Problem

When using OpenCode via ACP (e.g. IntelliJ IDEA), every session is titled ACP Session <random UUID>, making it impossible to distinguish sessions in the session list.
This happens because src/acp/session.ts hardcodes the title:

title: `ACP Session ${crypto.randomUUID()}`,

In contrast, TUI sessions get meaningful auto-generated titles based on the first message.

Expected Behavior

ACP sessions should get the same auto-titling treatment as TUI sessions — either:

1. Use a temporary placeholder on creation, then auto-generate a title from the first message (same as TUI)
2. Accept an optional title from the ACP client, falling back to auto-generation if not provided

Steps to Reproduce

1. Connect to OpenCode via ACP (e.g. from IntelliJ IDEA)
2. Start a conversation
3. Check the session list — title remains ACP Session <UUID>

Reference

packages/opencode/src/acp/session.ts — ACPSessionManager.create()