Description

When a non-assistant ACP session uses a custom workspace with enabledSkills but no presetContext, skills are completely lost.

Root Cause

1. Custom workspaces skip setupAssistantWorkspace symlinks to avoid polluting user directories
2. For native-support backends (claude, codex, etc.), the first-message handler only injected presetContext — it didn't check whether symlinks were actually created
3. Result: no symlinks + no prompt injection = skills lost

Fix

Check customWorkspace flag in addition to native skill support. When custom workspace is used, always fallback to prompt injection via prepareFirstMessageWithSkillsIndex.