Feature Request: Argument auto-completion for skills / custom commands

Describe the feature or problem you'd like to solve

Skills can expose multiple parameters, but users currently need to remember them or look up the skill documentation before invocation. It would be useful for Copilot CLI to support PowerShell-style argument auto-completion for skills and custom commands.

Proposed solution

Proposed behavior

At minimum:

Auto-complete declared parameter names.
Support completion from partial parameter names.
Avoid suggesting already-used, non-repeatable parameters.
Derive completions directly from the skill's parameter/schema definition.

Optionally, parameter metadata could enable value completion:

file → filesystem completion
enum → allowed values
boolean → true / false
Why this is useful
Improves discoverability of skill capabilities.
Reduces typos and invalid arguments.
Avoids repeatedly checking skill documentation.
Makes parameter-rich skills feel like first-class CLI commands.
Lets skill authors define arguments once, with the CLI providing the completion experience automatically.

Example prompts or workflows

/deploy-service --environment --config <config.json>

Additional context

No response

Inizia individuando il punto di ingresso del completamento di Copilot CLI e la definizione del parametro o dello schema di skill/custom command utilizzata per l’invocazione. Traccia il modo in cui vengono analizzati gli argomenti parziali, quindi verifica che vengano suggeriti i parametri dichiarati e non utilizzati e che il completamento dei valori facoltativi segua le regole dei metadati indicate.