Plugin agents not loaded from standalone agent-only plugins

Description

Agents defined in standalone plugins (plugins with only an agents/ directory and no skills/ directory) are not being registered as available subagent_type options for the Task tool.

Steps to Reproduce

1. Install a plugin that only has agents (no skills), e.g., code-simplifier@claude-plugins-official
2. Verify the plugin is installed: check ~/.claude/plugins/installed_plugins.json
3. Start a new Claude Code session
4. Attempt to use subagent_type: "code-simplifier:code-simplifier" in Task tool
5. The agent is not available

Expected Behavior

Agents from all installed plugins should be discovered and registered, regardless of whether the plugin also contains skills.

Actual Behavior

Only agents from plugins that ALSO have a skills/ directory are registered. For example:

• superpowers:code-reviewer ✅ works (plugin has both skills/ and agents/ directories)
• code-simplifier:code-simplifier ❌ not available (plugin only has agents/ directory)

Environment

• Claude Code version: 2.1.25
• OS: macOS

Plugin Structures (both valid)

Working plugin (superpowers):

superpowers/4.1.1/
├── .claude-plugin/
│   └── plugin.json
├── agents/
│   └── code-reviewer.md
└── skills/
    └── (multiple skill directories)

Non-working plugin (code-simplifier):

code-simplifier/1.0.0/
├── .claude-plugin/
│   └── plugin.json
└── agents/
    └── code-simplifier.md

Both have identical valid frontmatter in their agent .md files (name, description, model fields).

Workaround

Copy the agent .md file into a plugin that has skills:

cp ~/.claude/plugins/cache/claude-plugins-official/code-simplifier/1.0.0/agents/code-simplifier.md \
   ~/.claude/plugins/cache/claude-plugins-official/superpowers/4.1.1/agents/

Then use superpowers:code-simplifier instead.