Plugin skills: option to require namespace-qualified invocation

Feature Request

Add a SKILL.md frontmatter option to disable the short-name form of a plugin skill, requiring users to invoke it with the fully qualified /<plugin>:<skill> name.

Problem

When a plugin defines a skill with a generic name (e.g., add in an issues plugin), it registers as both /add and /issues:add. The short name /add is ambiguous and could collide with other plugins or project-level skills. There's currently no way to require the namespace prefix.

The existing frontmatter options don't cover this case:

| Option | Effect |
|---|---|
| (default) | Both /add and /issues:add work |
| disable-model-invocation: true | Claude can't auto-invoke, but user can still use /add |
| user-invocable: false | Hidden from user entirely — neither name works |

None of these allow: "user can invoke, but only via the qualified name."

Proposed Solution

A new frontmatter field, e.g.:

---
name: add
description: Create a GitHub issue
require-namespace: true
---

When require-namespace: true is set:

• /issues:add works normally
• /add is not registered as a standalone skill
• The skill does not appear in autocomplete without the namespace prefix

Use Case

Plugin authors who choose descriptive-but-generic skill names (add, list, run, check) that only make sense in the context of their plugin namespace. The qualified form (/issues:add, /tests:run) is clear; the short form is not.

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