**Planning boids simulation**

I need to create a self-contained index.html file that includes both CSS and JS for a full-screen boids flocking simulation. The simulation should feature boids represented as triangles that can smoothly steer away from the mouse cursor. This requires implementing behavior rules like separation, alignment, and cohesion. I'll ensure everything runs efficiently in a sandbox environment and focus on performance, using requestAnimationFrame and canvas for smooth animations. I'll prepare to write all necessary content using the appropriate tools.**Designing boid behavior**

I'm planning to implement the boids as oriented triangles that rotate to match their velocity angle. For drawing, I'll use the canvas context methods to save, translate, and rotate before drawing the triangle. The canvas will be full-screen, adjusting to the window size and handling resizing. I want the boids to flee from the mouse cursor, so I’ll track mouse coordinates and perhaps allow clicking and dragging to influence their movement. There might also be a simple UI, and I’ll ensure to add performance optimizations as needed.**Creating triangle drawing for boids**

I need to set up the canvas to draw triangles, starting with beginPath and moving to the appropriate position for the first point. I’ll define the triangle shape points and incorporate rotation. For smooth steering, I need to limit maxForce and calculate the steering vector. The code should avoid any network dependency and remain simple, so I'll set the HTML structure with a full-screen canvas. I'll include the Boid class and manage an array of around 120 boids for performance. The design is coming together nicely!**Building full-screen flocking simulation**

The user requested a full-screen flocking simulation, so I need to ensure everything is optimized for larger displays. I'll set up a Vector2 helper class that allows for 2D vector operations, but I might implement this using simple objects for efficiency. 

Next, I’ll create a wrap-around function to manage boids' positions, allowing them to reappear on the opposite side of the canvas. For the boid objects, I’ll keep their properties like position, velocity, and acceleration in arrays without a separate class. I'll implement the necessary update and flocking functions as discussed, ensuring good performance, even for around 200 boids.**Implementing neighbor detection**

I’m setting up simple neighbor detection using a perception radius. For this, I’ll create three core functions: separation, alignment, and cohesion. Each function will compute the necessary steering behaviors for the boids based on their positions and velocities. 

I'll also implement a flee behavior from the mouse if it's within a certain radius. The triangles representing the boids will be drawn on a gradient background, creating trails by allowing some transparency. Lastly, I'll ensure the performance by effectively managing the boid behavior calculations and responding to window resizing.**Updating boid behaviors and drawing**

In the update loop, I’m going through each boid to apply separation, alignment, and cohesion behaviors. I’ll also add a flee behavior from the mouse if it’s within a certain proximity. 

Then, I’ll update each boid's velocity and position while ensuring they wrap around the screen edges. For drawing, I’ll create an oriented triangle representing each boid. The background will have a subtle gradient for trails. Additionally, I need to implement high-DPI support using devicePixelRatio for crisp rendering. 

Finally, I’ll package everything into an `index.html` file.

**Confirming simulation setup**

I've created an index.html that features a full-screen flocking simulation of boids, using separation, alignment, and cohesion. The boids are represented by triangles that smoothly steer and flee from the mouse cursor, and there are 180 of them. This single-page app is self-contained, with no external resources required. To use it, just open the index.html in a browser, and it runs full screen while allowing you to move your mouse to see the flee effect.**Summarizing features and customization options**

I can provide a quick summary of features for the boids simulation. You can adjust the number of boids and their perception radius easily since the code is appended to the index.html in the current directory. Next steps include customizing parameters, adding color, and tweaking features. The file is ready, and I’ll confirm its location while also giving a brief tip about performance, like adjusting the number of boids for smoother visuals. Pressing Escape can be used to exit.**Confirming simulation setup and usage**

I’m confirming that the index.html has been created in the /tmp/ws directory, featuring full-screen boid triangles. To use it, just open the file in a modern browser and move your mouse—it'll flee from the cursor when active. For customization, you can tweak constants like BOID_COUNT, MAX_SPEED, and others. To ensure it’s truly fullscreen, the body uses overflow hidden. You can also adjust visuals like triangle color and size. If you'd like to run it offline, just open the file. For further improvements, consider adding color variations based on velocity or ID.**Finalizing output details**

I need to confirm that there’s no leakage of internal tools. I've already used the write tool to create the index.html file, which is now located at /tmp/ws/index.html. I'll provide a relative path and include a brief content summary to keep things concise. The final answer will confirm the file's presence and summarize its contents clearly so users understand what to expect without unnecessary details.