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.

Start with the explicit slash-command invocation path and the separate automatic skill lookup or registry path, using disable-model-invocation and /implement #1147 as search terms. Reproduce the issue on Copilot CLI 1.0.80, then verify that an explicitly invoked skill executes without a duplicate Skill not found message while automatic invocation remains disabled.