Plugin agents not loading in v2.1.12

Bug Description

Plugin agents are not appearing in the available agents list after updating to v2.1.12.

Steps to Reproduce

1. Install a plugin with agents (e.g. code-simplifier from claude-plugins-official)
2. Enable it in settings
3. Try to use Task tool with subagent_type: "code-simplifier"
4. Error: "Agent type 'code-simplifier' not found"

Expected Behavior

Plugin agents should appear in the available agents list and be invokable via Task tool.

Actual Behavior

Only built-in agents and ~/.claude/agents/ agents load. Plugin agents from ~/.claude/plugins/ are not registered.

Evidence

• Plugin installed: ~/.claude/plugins/installed_plugins.json shows code-simplifier@1.0.0
• Plugin enabled: ~/.claude/settings.json has "code-simplifier@claude-plugins-official": true
• Agent file exists: ~/.claude/plugins/cache/claude-plugins-official/code-simplifier/1.0.0/agents/code-simplifier.md
• Worked in v2.1.11, broken in v2.1.12

Environment

• Claude Code v2.1.12
• macOS
• Regression from v2.1.11