use native Claude Code installer (curl | bash), check prerequisites in birth

- install-cli.sh: suggest curl -sL https://claude.ai/install.sh | bash
- birth.sh: check colony, colony-agent, claude are in PATH before starting
- Fail fast with clear error messages if anything is missing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/install-cli.sh

@@ -35,7 +35,7 @@ echo "colony:       $(${INSTALL_DIR}/colony --version 2>/dev/null || echo 'insta
 echo "colony-agent: $(${INSTALL_DIR}/colony-agent --version 2>/dev/null || echo 'installed')"
 echo ""
 echo "Next: install Claude Code if not already installed:"
-echo "  npm install -g @anthropic-ai/claude-code"
+echo "  curl -sL https://claude.ai/install.sh | bash"
 echo ""
 echo "Then birth an agent:"
 echo "  sudo bash scripts/birth.sh scout 'help with research'"