Copilot installs agent files as gsd-*.agent.md (not gsd-*.md), so
checkAgentsInstalled() always returned agents_installed=false for Copilot.
- checkAgentsInstalled() now recognises both .md and .agent.md formats
- getAgentsDir() respects GSD_AGENTS_DIR env override for testability
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>