feat(summon): auto-include markdown supporting files when loading skills

Supporting files were only listed as paths with "use file tools to read
these" — but environments without the developer extension (e.g. slackbots)
have no file-reading tools, making reference files unreachable. Now .md/.mdx
supporting files are read and included inline alongside the SKILL.md content,
consistent with how SKILL.md itself is already read and included. Non-markdown
files continue to be listed as paths.

Author: wpfleger96

crates/goose/src/agents/platform_extensions/summon.rs

@@ -1054,18 +1054,40 @@ impl SummonClient {
                 );
 
                 if !source.supporting_files.is_empty() {
-                    output.push_str(&format!(
-                        "\n## Supporting Files\n\nSkill directory: {}\n\nThe following supporting files are available:\n",
-                        source.path.display()
-                    ));
+                    let mut other_files: Vec<std::path::PathBuf> = Vec::new();
+
                     for file in &source.supporting_files {
+                        let is_md = file
+                            .extension()
+                            .map_or(false, |ext| ext == "md" || ext == "mdx");
+
                         if let Ok(relative) = file.strip_prefix(&source.path) {
+                            if is_md {
+                                if let Ok(file_content) = std::fs::read_to_string(file) {
+                                    output.push_str(&format!(
+                                        "\n## {}\n\n{}\n",
+                                        relative.display(),
+                                        file_content.trim()
+                                    ));
+                                }
+                            } else {
+                                other_files.push(relative.to_path_buf());
+                            }
+                        }
+                    }
+
+                    if !other_files.is_empty() {
+                        output.push_str(&format!(
+                            "\n## Other Files\n\nSkill directory: {}\n\n",
+                            source.path.display()
+                        ));
+                        for relative in &other_files {
                             output.push_str(&format!("- {}\n", relative.display()));
                         }
+                        output.push_str(
+                            "\nUse the file tools to read these files or run scripts as directed.\n",
+                        );
                     }
-                    output.push_str(
-                        "\nUse the file tools to read these files or run scripts as directed.\n",
-                    );
                 }
 
                 output.push_str("\n---\nThis knowledge is now available in your context.");