Slash commands for plugin skills return 'Unknown skill' with custom CLAUDE_CONFIG_DIR

Description

When using a custom profile via CLAUDE_CONFIG_DIR (e.g., CLAUDE_CONFIG_DIR=~/.claude-custom claude), slash commands for plugin-provided skills return "Unknown skill", even though the skill is correctly loaded and functional via the Skill tool and natural language triggering.

Steps to Reproduce

1. Set up a custom profile:

``bash
export CLAUDE_CONFIG_DIR=~/.claude-custom
``

1. Install a plugin from a custom marketplace:

``bash
claude plugin install my-skill@my-marketplace
``

1. Launch Claude Code with the custom profile and run /reload-plugins — output shows the plugin is loaded:

``
Reloaded: 3 plugins · 0 commands · 5 agents · 0 hooks · 0 plugin MCP servers
``

1. The skill appears in the system prompt's available skills list:

```

• my-plugin:my-skill: ...description...

```

1. Try invoking via slash command:

``
/my-plugin:my-skill
``
Result: "Unknown skill"

Expected Behavior

/my-plugin:my-skill should invoke the skill, just like the Skill tool does.

What Works

• Skill tool invocation: Skill tool with skill: "my-plugin:my-skill" works correctly
• Natural language triggering: Describing the task in natural language correctly triggers the skill
• Plugin list: claude plugin list shows the plugin as installed and enabled
• Plugin validation: claude plugin validate <path> passes

Environment

• OS: macOS (Darwin 25.3.0, Apple Silicon)
• Shell: zsh
• Custom profile via CLAUDE_CONFIG_DIR (not the default ~/.claude/)
• Plugin installed from a custom marketplace (extraKnownMarketplaces in settings.json)

Analysis

The slash command autocomplete/resolution system and the Skill tool appear to use different lookup paths. The Skill tool correctly resolves plugin skills from the custom CLAUDE_CONFIG_DIR, but the interactive slash command resolver may be falling back to the default ~/.claude/ directory, missing the plugin registry in the custom profile.