Live, data-driven argument autocomplete for custom slash commands/skills

Custom slash commands and skills support a static argument-hint in frontmatter, but there's no way to make argument suggestions dynamic or filtered live as the user types. Requesting: a mechanism (e.g. a script/file a skill can point to) that the / command-input dropdown queries to produce live, prefix-filtered suggestions for arguments -- similar to iOS predictive-text-style autocomplete -- instead of only a fixed hint string.

Concrete use case: a skill maintains a small JSON cache of named presets. Typing /mycommand go should be able to surface /mycommand godot_intermediate_coder from that cache, filtered as the user types, rather than requiring the user to remember exact preset names or run a separate list command.

Start by tracing the command-input dropdown and the custom slash command/skill argument handling; the issue does not name specific files or tests. Define how a skill points to a suggestion source, how suggestions are queried and prefix-filtered while typing, and how the JSON-cache use case is represented; done means matching argument suggestions appear live instead of only the static argument-hint.