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/types/copilot.ts

@@ -0,0 +1,31 @@
+export type CopilotAgent = {
+  name: string
+  content: string
+}
+
+export type CopilotGeneratedSkill = {
+  name: string
+  content: string
+}
+
+export type CopilotSkillDir = {
+  name: string
+  sourceDir: string
+}
+
+export type CopilotMcpServer = {
+  type: string
+  command?: string
+  args?: string[]
+  url?: string
+  tools: string[]
+  env?: Record<string, string>
+  headers?: Record<string, string>
+}
+
+export type CopilotBundle = {
+  agents: CopilotAgent[]
+  generatedSkills: CopilotGeneratedSkill[]
+  skillDirs: CopilotSkillDir[]
+  mcpConfig?: Record<string, CopilotMcpServer>
+}