feat(skills): support `$`-triggered picker for user-invocable skills

[Morphone0429]

Summary

Crush already supports manually invocable skills through the command palette (Ctrl+P) when a skill declares user-invocable: true.

It would be useful to also support a lightweight, composer-local skill picker triggered by typing $.

Proposed behavior

In the interactive composer:

1. Type $ to open a filtered catalog of user-invocable skills.
2. Continue typing to filter skill names.
3. Select a skill with the keyboard; represent the selected skill visibly in the pending prompt.
4. Submit the accompanying request to load that skill for the current turn.
5. Esc dismisses the picker.

Global skills could retain the existing user:<name> convention and project skills project:<name>.

Scope

• Keep Ctrl+P as the existing command-palette entry point.
• Only offer skills explicitly marked user-invocable: true.
• Do not intercept ordinary literal $ text, shell snippets, or configuration syntax when the composer context does not match a skill-selection interaction.

Motivation

This would make frequently used skills easier to discover and invoke without leaving the active prompt. The interaction is familiar in some agent CLIs, but the request is limited to a small optional composer shortcut rather than changing the existing skills model.