feat: Add GitHub Copilot converter target

Add Copilot as the 6th converter target, transforming Claude Code plugins
into Copilot's native format: custom agents (.agent.md), agent skills
(SKILL.md), and MCP server configuration JSON.

Component mapping:
- Agents → .github/agents/{name}.agent.md (with Copilot frontmatter)
- Commands → .github/skills/{name}/SKILL.md
- Skills → .github/skills/{name}/ (copied as-is)
- MCP servers → .github/copilot-mcp-config.json
- Hooks → skipped with warning

Also adds `compound sync copilot` support and fixes YAML quoting for
the `*` character in frontmatter serialization.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/utils/frontmatter.ts

@@ -58,7 +58,7 @@ function formatYamlValue(value: unknown): string {
   if (raw.includes("\n")) {
     return `|\n${raw.split("\n").map((line) => `  ${line}`).join("\n")}`
   }
-  if (raw.includes(":") || raw.startsWith("[") || raw.startsWith("{")) {
+  if (raw.includes(":") || raw.startsWith("[") || raw.startsWith("{") || raw === "*") {
     return JSON.stringify(raw)
   }
   return raw