Plugin-bundled skills not discovered by skill registry

Bug Description

Skills bundled inside Claude Code plugins (installed via claude plugin install) are not discovered by the skill registry, except for skills that are also independently installed via npx skills into ~/.claude/skills/ or ~/.agents/skills/.

Steps to Reproduce

1. Install the superpowers plugin: claude plugin install superpowers@claude-plugins-official
2. Verify it's installed and enabled: claude plugin list shows superpowers@claude-plugins-official v5.0.5, enabled
3. Start a new Claude Code session
4. Try to invoke any bundled skill (e.g., /brainstorming) — returns "Unknown skill"

Expected Behavior

All 14 skills in the plugin's skills/ directory should be discoverable and invocable:

• brainstorming, dispatching-parallel-agents, executing-plans, finishing-a-development-branch, receiving-code-review, requesting-code-review, subagent-driven-development, systematic-debugging, test-driven-development, using-git-worktrees, using-superpowers, verification-before-completion, writing-plans, writing-skills

Actual Behavior

Only using-superpowers is discovered — because it happens to also be installed as a standalone skill in ~/.agents/skills/using-superpowers/ (via npx skills add). The other 13 skills exist on disk in the plugin cache but are invisible to the skill registry.

Investigation

• Plugin install path: ~/.claude/plugins/cache/claude-plugins-official/superpowers/5.0.5/
• All 14 skills exist under skills/ in that path with valid YAML frontmatter (name:, description:)
• Skill discovery appears to scan ~/.claude/skills/ and ~/.agents/skills/ but does not scan plugin cache directories (~/.claude/plugins/cache/*/)
• The plugin's package.json is minimal (no explicit skills manifest), so discovery likely relies on directory scanning convention

Environment

• macOS (Darwin 24.5.0)
• Claude Code (latest as of 2026-03-20)
• Plugin: superpowers@claude-plugins-official v5.0.5