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.

Inizia con il percorso di invocazione esplicita del comando slash e con il percorso separato di ricerca o registrazione automatica delle skill, usando disable-model-invocation e /implement #1147 come termini di ricerca. Riproduci il problema su Copilot CLI 1.0.80, quindi verifica che una skill invocata esplicitamente venga eseguita senza un messaggio Skill not found duplicato, mentre l'invocazione automatica rimane disabilitata.