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/sync/copilot.ts

@@ -0,0 +1,100 @@
+import fs from "fs/promises"
+import path from "path"
+import type { ClaudeHomeConfig } from "../parsers/claude-home"
+import type { ClaudeMcpServer } from "../types/claude"
+import { forceSymlink, isValidSkillName } from "../utils/symlink"
+
+type CopilotMcpServer = {
+  type: string
+  command?: string
+  args?: string[]
+  url?: string
+  tools: string[]
+  env?: Record<string, string>
+  headers?: Record<string, string>
+}
+
+type CopilotMcpConfig = {
+  mcpServers: Record<string, CopilotMcpServer>
+}
+
+export async function syncToCopilot(
+  config: ClaudeHomeConfig,
+  outputRoot: string,
+): Promise<void> {
+  const skillsDir = path.join(outputRoot, "skills")
+  await fs.mkdir(skillsDir, { recursive: true })
+
+  for (const skill of config.skills) {
+    if (!isValidSkillName(skill.name)) {
+      console.warn(`Skipping skill with invalid name: ${skill.name}`)
+      continue
+    }
+    const target = path.join(skillsDir, skill.name)
+    await forceSymlink(skill.sourceDir, target)
+  }
+
+  if (Object.keys(config.mcpServers).length > 0) {
+    const mcpPath = path.join(outputRoot, "copilot-mcp-config.json")
+    const existing = await readJsonSafe(mcpPath)
+    const converted = convertMcpForCopilot(config.mcpServers)
+    const merged: CopilotMcpConfig = {
+      mcpServers: {
+        ...(existing.mcpServers ?? {}),
+        ...converted,
+      },
+    }
+    await fs.writeFile(mcpPath, JSON.stringify(merged, null, 2), { mode: 0o600 })
+  }
+}
+
+async function readJsonSafe(filePath: string): Promise<Partial<CopilotMcpConfig>> {
+  try {
+    const content = await fs.readFile(filePath, "utf-8")
+    return JSON.parse(content) as Partial<CopilotMcpConfig>
+  } catch (err) {
+    if ((err as NodeJS.ErrnoException).code === "ENOENT") {
+      return {}
+    }
+    throw err
+  }
+}
+
+function convertMcpForCopilot(
+  servers: Record<string, ClaudeMcpServer>,
+): Record<string, CopilotMcpServer> {
+  const result: Record<string, CopilotMcpServer> = {}
+  for (const [name, server] of Object.entries(servers)) {
+    const entry: CopilotMcpServer = {
+      type: server.command ? "local" : "sse",
+      tools: ["*"],
+    }
+
+    if (server.command) {
+      entry.command = server.command
+      if (server.args && server.args.length > 0) entry.args = server.args
+    } else if (server.url) {
+      entry.url = server.url
+      if (server.headers && Object.keys(server.headers).length > 0) entry.headers = server.headers
+    }
+
+    if (server.env && Object.keys(server.env).length > 0) {
+      entry.env = prefixEnvVars(server.env)
+    }
+
+    result[name] = entry
+  }
+  return result
+}
+
+function prefixEnvVars(env: Record<string, string>): Record<string, string> {
+  const result: Record<string, string> = {}
+  for (const [key, value] of Object.entries(env)) {
+    if (key.startsWith("COPILOT_MCP_")) {
+      result[key] = value
+    } else {
+      result[`COPILOT_MCP_${key}`] = value
+    }
+  }
+  return result
+}