Copilot CLI v1.0.11 reads `additionalContext` from sessionStart hook
output, but the session-start script only emits the Claude Code-specific
nested format. Add COPILOT_CLI env var detection so Copilot CLI gets the
SDK-standard top-level `additionalContext` while Claude Code continues
getting `hookSpecificOutput`.
Based on PR #910 by @culinablaz.