feat(tools): auto-deliver write_file results as channel attachments

Flip write_file deliver param default from false to true so result files
(reports, articles, generated content) are automatically sent as document
attachments to chat channels without requiring explicit LLM opt-in.
Add .md MIME type to mimeFromExt for proper markdown file delivery.

internal/agent/loop_media.go

@@ -98,6 +98,8 @@ func mimeFromExt(ext string) string {
 		return "application/msword"
 	case ".xls", ".xlsx":
 		return "application/vnd.ms-excel"
+	case ".md":
+		return "text/markdown"
 	default:
 		return "application/octet-stream"
 	}

internal/tools/filesystem_write.go

@@ -77,7 +77,7 @@ func (t *WriteFileTool) Parameters() map[string]any {
 			},
 			"deliver": map[string]any{
 				"type":        "boolean",
-				"description": "If true, deliver this file to the user as an attachment (image, document, etc.)",
+				"description": "Deliver this file to the user as an attachment. Defaults to true. Set to false for intermediate/temporary files (e.g. config, cache, temp scripts).",
 			},
 		},
 		"required": []string{"path", "content"},
@@ -87,7 +87,10 @@ func (t *WriteFileTool) Parameters() map[string]any {
 func (t *WriteFileTool) Execute(ctx context.Context, args map[string]any) *Result {
 	path, _ := args["path"].(string)
 	content, _ := args["content"].(string)
-	deliver, _ := args["deliver"].(bool)
+	deliver := true
+	if v, ok := args["deliver"].(bool); ok {
+		deliver = v
+	}
 	if path == "" {
 		return ErrorResult("path is required")
 	}