Support parameter declarations for custom plugin skills

Problem

Built-in skills (e.g. /review) display argument hints like <pr-url> in the prompt, giving users a clear indication of what input is needed. Custom plugin skills defined as markdown files have no way to declare required parameters, so they can't provide the same UX. Instead, the skill expands into instructions and the agent has to ask the user for the missing argument in a follow-up — an extra round-trip.

Proposal

Add an optional parameters field to the plugin skill format so custom skills can declare their expected arguments. For example, in plugin.json or a skill manifest:

{
  "skills": {
    "check-cluster-state": {
      "parameters": [
        {
          "name": "cluster-name",
          "description": "The NKE cluster to check",
          "required": true,
          "placeholder": "<cluster-name>"
        }
      ]
    }
  }
}

This would enable:

• Displaying placeholder hints in the prompt (e.g. /check-cluster-state <cluster-name>)
• Tab-completion or validation of required arguments before skill expansion
• Parity between built-in and custom plugin skills

Current Workaround

Users can append arguments inline (e.g. /check-cluster-state cluster: nke-site-forge-az24-dev6) and the agent will pick them up, but there's no discoverability or prompting for this.