Plugin skills: standalone registration removes namespace discoverability and namespace form lacks argument-hint

Problem

Plugin skills have a tradeoff that makes them less discoverable:

Option A - Standalone registration (e.g., /my-skill):

• Shows argument-hint/signature in autocomplete
• But the skill disappears from the namespace prefix list (/my-plugin: no longer shows it)

Option B - No standalone registration (e.g., /my-plugin:my-skill):

• Browsable by typing /my-plugin: to see all plugin skills
• But no argument-hint/signature is displayed

There is no way to get both behaviors simultaneously.

Expected Behavior

1. Plugin skills registered as standalone should still appear under their namespace prefix (e.g., /my-plugin:my-skill should remain discoverable even when /my-skill exists as standalone)
2. Namespace-prefixed skills (/plugin-name:skill-name) should display the argument-hint from the skill's SKILL.md frontmatter, just like standalone skills do

Steps to Reproduce

1. Create a plugin with a skill that has argument-hint in its SKILL.md frontmatter
2. Register it as a standalone skill (via marketplace.json + skills/ directory)
3. Type /skill-name - argument-hint shows correctly
4. Type /plugin-name: - the skill no longer appears in the list
5. Remove standalone registration
6. Type /plugin-name: - the skill appears again, but without argument-hint

Use Case

When a user forgets a skill name but remembers which plugin it belongs to, they should be able to type /plugin-name: to browse available skills. Currently this only works if the skill is NOT registered standalone, forcing a choice between discoverability and signature display.

Environment

• Claude Code CLI on Windows 11
• Observed with both custom and first-party plugins