feat(proxy): make Copilot API target configurable for enterprise environments

[Copilot]

When running in GHEC/GHES environments, the api-proxy sidecar was hardcoded to route Copilot traffic to api.githubcopilot.com, breaking enterprise deployments that require api.enterprise.githubcopilot.com.

Changes

• Auto-derivation: api-proxy now detects non-github.com GITHUB_SERVER_URL and automatically routes to api.enterprise.githubcopilot.com
• Explicit override: COPILOT_API_TARGET env var and --copilot-api-target CLI flag allow overriding the target hostname
• Input validation: COPILOT_API_TARGET is normalized — full URLs are parsed to extract hostname, bare hostnames are validated to reject paths/whitespace
• Env forwarding: GITHUB_SERVER_URL and GITHUB_API_URL are forwarded from the host into both the agent and api-proxy containers
• Precedence: COPILOT_API_TARGET > derived from GITHUB_SERVER_URL > api.githubcopilot.com

Target derivation logic (api-proxy)

// COPILOT_API_TARGET wins if set
// Otherwise, non-github.com GITHUB_SERVER_URL → enterprise endpoint
// Otherwise, public default
function deriveCopilotApiTarget() {
  const envTarget = process.env.COPILOT_API_TARGET;
  if (envTarget) return normalizeHostname(envTarget);

  const serverUrl = process.env.GITHUB_SERVER_URL;
  if (serverUrl) {
    const hostname = new URL(serverUrl).hostname;
    if (hostname !== 'github.com') return 'api.enterprise.githubcopilot.com';
  }

  return 'api.githubcopilot.com';
}

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.