Feature: @skill autocomplete with template snippets

Feature Request

What: When typing @ in the Claude Code input, show a fuzzy-searchable dropdown of available skills (from plugins, marketplace, and user-defined). Selecting a skill pastes a template prompt with placeholder fields to fill in.

Why: Skills from the marketplace (e.g., @tdd-guide, @senior-architect) already work via text pattern matching — the model sees @skill-name and loads the corresponding skill. But there's no discoverability or templating. Users have to remember skill names and know the expected input format.

How it could work:

1. Plugin SKILL.md files already have YAML frontmatter with name: and triggers:
2. On @ keypress, Claude Code reads registered skills and shows a picker (like VS Code's @ symbol picker)
3. Each skill can optionally define a template: field in frontmatter — a fill-in-the-blanks prompt
4. Selecting a skill pastes @skill-name\n<template> into the input
5. Cursor lands on the first [PLACEHOLDER] for easy tab-through

Example flow:

User types: @tdd
Dropdown shows: tdd-guide | Test-driven development with Jest, Pytest, JUnit...
User selects tdd-guide
Input becomes:

@tdd-guide

Requirements:
- [REQUIREMENT_1]
- [REQUIREMENT_2]

Language: [TypeScript / Python / Java]
Framework: [Jest / Pytest / JUnit / Vitest]

Current workaround: I built a shell helper (sk) that uses fzf + templates + pbcopy, but native integration would be much better.

Frontmatter extension needed:

---
name: "tdd-guide"
description: "Test-driven development..."
triggers:
  - generate tests
  - TDD workflow
template: |
  Requirements:
  - [REQUIREMENT_1]
  - [REQUIREMENT_2]
  Language: [TypeScript / Python / Java]
  Framework: [Jest / Pytest / JUnit / Vitest]
---

This would make the entire skill ecosystem more discoverable and lower the barrier to using skills correctly.