Skills UI: frontmatter JSON field is unlabeled and not human-friendly

Problem

When creating or editing a skill in the Claude Code UI, there's a "frontmatter" field that takes raw JSON. Two issues:

1. It's not clear what it's for. There's no inline help, example, or link explaining what frontmatter does, what keys are valid, or how it affects skill behavior. A user encountering this field for the first time has no way to know whether to fill it in, leave it blank, or what shape the JSON should take.

1. Raw JSON is a bad input format for humans. Even if the underlying storage/API is JSON, the UI should not require users to hand-edit JSON syntax — quoting keys, balancing braces, escaping strings. This is error-prone and unfriendly compared to a structured form (one row per key/value, dropdowns for known fields, etc.).

Suggestions

• Replace the JSON textarea with a structured editor: known frontmatter keys (name, description, etc.) get dedicated labeled inputs; unknown keys can still be added as ad-hoc rows.
• Add a tooltip/help link explaining what frontmatter is and what fields are recognized.
• If keeping the raw JSON view as an "advanced" option, default to the structured form.

Impact

Lower friction for authoring skills, fewer malformed-JSON errors, more discoverable feature.