[Bug] Plugin manifest 'agents' field validation rejects valid path with .claude prefix

Bug Description

Plugin manifest validation rejects "agents": "./.claude/agents" with the unhelpful error message "Validation errors: agents: Invalid input". The path is valid and the directory exists, but the validator doesn't accept it.

The /plugin UI shows the error on the Errors tab but provides no guidance on what format is expected.

Version

Claude Code 2.1.92, Windows 11

Steps to Reproduce

1. Create a plugin with an agents field pointing to a .claude/ subdirectory:

{
  "name": "my-plugin",
  "version": "1.0.1",
  "skills": "./skills",
  "commands": "./commands",
  "agents": "./.claude/agents"
}

1. Place agent .md files in .claude/agents/ directory.
2. Install the plugin via marketplace.
3. Open /plugin → Errors tab.

Expected Behavior

• Plugin loads successfully since the path is valid and the directory/files exist
• OR: error message explains the expected format (e.g., "agents must be a string path not starting with .claude" or "agents must be an array of paths")

Actual Behavior

• Error tab shows: Plugin my-plugin has an invalid manifest file at <path>. Validation errors: agents: Invalid input
• No indication of what format is valid
• The plugin's agents are not loaded

Workaround

Move the agents directory from .claude/agents/ to ./agents/ and update the manifest to "agents": "./agents". This resolves the validation error.

Notes

The plugin-dev manifest reference shows agents can be a string ("./agents") or array (["./agents", "./specialized-agents"]). The docs don't mention any restriction on .claude/ prefixed paths. If this is intentional (e.g., .claude/ is reserved), the error message should say so.