Third-party marketplace plugins cannot register skills/commands/agents

Bug Description

Plugins installed from third-party marketplaces (any marketplace other than claude-plugins-official) show as installed and enabled in the Manage Plugins UI (green dot, toggle on) but fail to register any skills, commands, or agents. The / command autocomplete shows "No matching commands" for the plugin prefix.

Hooks and CLAUDE.md from the same plugin DO load correctly — only skill/command/agent discovery is broken.

Steps to Reproduce

1. Create a third-party marketplace (GitHub repo with .claude-plugin/marketplace.json)
2. Add a plugin with valid skills/ directory containing properly formatted SKILL.md files
3. Install: Add marketplace via Manage Plugins → Marketplaces tab → Add
4. Enable the plugin (toggle on, green dot appears)
5. Restart VS Code / Claude Code
6. Type /<plugin-name> — shows "No matching commands"

Expected Behavior

Skills from third-party marketplace plugins should appear in the / command autocomplete, the same way skills from claude-plugins-official plugins appear.

Actual Behavior

• Plugin shows as installed and enabled (green dot) ✅
• Plugin hooks load and fire correctly ✅
• Plugin CLAUDE.md loads into context ✅
• Plugin skills/commands/agents are NOT registered ❌
• / autocomplete shows "No matching commands" for the plugin prefix ❌

Environment

• Claude Code version: 2.1.76
• VS Code extension
• macOS Darwin 24.3.0
• Plugin: sia@sia-plugins (marketplace: rkarim08/sia)

Diagnostic Evidence

1. Every working plugin on the system is from claude-plugins-official (20 plugins)
2. The only non-official plugin (sia@sia-plugins) has zero skills registered
3. Plugin cache structure is correct and matches working plugins (verified by comparing against superpowers@claude-plugins-official)
4. SKILL.md frontmatter is valid (name, description fields present, correct format)
5. Tested with minimal plugin (1 skill, no hooks, no CLAUDE.md) — still no registration
6. installed_plugins.json correctly points to cache, settings.json has "sia@sia-plugins": true

Plugin Structure (matches working plugins exactly)

.claude-plugin/
├── marketplace.json    ← name: "sia-plugins", plugins: [{name: "sia", source: "./"}]
└── plugin.json         ← name: "sia", version: "1.0.12"
skills/
├── sia-stats/
│   └── SKILL.md        ← name: sia-stats, description: "Show SIA knowledge graph statistics"
├── sia-search/
│   └── SKILL.md
└── ... (47 skills total)
hooks/
└── hooks.json          ← Valid events only (PreToolUse, PostToolUse, etc.)

Workaround

None found. --plugin-dir flag was not tested successfully in VS Code extension context.

Hypothesis

The plugin loader may have special-case handling for claude-plugins-official that doesn't apply to third-party marketplaces — specifically in the skill/command/agent discovery phase. Hooks and CLAUDE.md use a different loading path that works for all marketplaces.