Avoid duplicate skill lookup for slash-invoked skills with disable-model-invocation

Summary

A skill that is explicitly invoked with a slash command can execute and inject its instructions into context, while another lookup path reports Skill not found, creating misleading context noise.

Reproduction

Use a skill with this frontmatter:

---
name: implement
description: "Implement a piece of work based on a spec or set of tickets."
disable-model-invocation: true
---

The local skill is C:\Users\omzig\.copilot\skills\implement\SKILL.md; the relevant setting is line 4: disable-model-invocation: true.

Then run:

/implement #1147

Observed behavior:

• The /implement workflow is recognized and its instructions are injected into context.
• A separate skill lookup reports: Skill not found: implement.
• The command can continue, but the error is misleading and makes it appear that the skill failed to load.

Environment: Copilot CLI 1.0.80 on Windows.

Expected behavior

disable-model-invocation: true should prevent automatic/model-driven invocation. It should not make an explicit slash-command invocation unavailable. When the explicit command path succeeds, the runtime should not perform a second conflicting lookup or emit Skill not found.

Suggested fix

Distinguish explicit slash-command invocation from automatic skill invocation. Resolve the command/skill once, honor disable-model-invocation only for automatic loading, and suppress the registry error when the explicit command path has already loaded the skill successfully.

This is primarily a context-quality issue: the implementation workflow works, but the duplicate/conflicting lookup adds noise and obscures the actual result.

从显式斜杠命令调用路径和独立的自动 skill 查找或注册路径入手，使用 disable-model-invocation 和 /implement #1147 作为搜索词。在 Copilot CLI 1.0.80 上复现该问题，然后验证显式调用的 skill 能够执行且不会出现重复的 Skill not found 消息，同时自动调用仍处于禁用状态。