Expand T-shirt section with new details and gallery

Added details about T-shirt editions, features, design gallery, and size chart.

Author: shenxianpeng

docs/merch.md

@@ -4,6 +4,66 @@ We are not selling T-shirts.
 
 The T-shirt is our way of saying thank you to people who support the project. If your cumulative donations on [Open Collective](https://opencollective.com/cpp-linter) reach **USD 100**, we will reach out personally and send you one.
 
+## About These T-Shirts
+
+These aren't for sale — just something we made for ourselves to wear and promote this open source project. If you've used cpp-linter, appreciate the open source approach, or want to support the project, consider getting one.
+
+<div class="grid cards" markdown>
+
+<div markdown>
+
+### White Edition
+
+![White T-shirt Front](static/merch/white-front.jpg){: style="max-width: 100%; border-radius: 8px;"}
+
+**Features:**
+
+- "cpp linter" logo with circuit board design
+- "Lint your C/C++ code in workflow" tagline
+
+</div>
+
+<div markdown>
+
+### Black Edition
+
+![Black T-shirt Front](static/merch/black-front.jpg){: style="max-width: 100%; border-radius: 8px;"}
+
+**Features:**
+
+- "cpp-linter" logo with circuit board design
+- "Lint your C/C++ code in workflow" tagline
+
+</div>
+
+</div>
+
+### Design Gallery
+
+<div class="image-grid" markdown>
+
+![White T-shirt](static/merch/white-demo.jpg){: style="max-width: 100%; border-radius: 8px;"}
+
+![Black T-shirt](static/merch/black-demo.jpg){: style="max-width: 100%; border-radius: 8px;"}
+
+</div>
+
+### Size Chart
+
+!!! warning "Important: Sizes run small!"
+    Please carefully check the size chart before ordering. **No returns or refunds** after shipping.
+
+| Size | S | M | L | XL | 2XL | 3XL | 4XL | 5XL |
+|------|---|---|---|----|----|-----|-----|-----|
+| **Chest (cm)** | 45 | 47 | 49 | 51 | 53 | 55 | 57 | 59 |
+| **Length (cm)** | 63 | 65 | 67 | 69 | 71 | 73 | 75 | 77 |
+| **Sleeve (cm)** | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 |
+| **Shoulder (cm)** | 40 | 42 | 44 | 46 | 48 | 50 | 52 | 54 |
+| **Height (cm)** | 155 | 160 | 165 | 170 | 175 | 180 | 185 | 190 |
+| **Weight (kg)** | 50± | 55± | 60± | 65± | 75± | 85± | 95± | 105± |
+
+*Note: Measurements are manual and may vary by 1-3cm. Everyone's body is different, so height and weight are for reference only. Since this is a custom product, sizes cannot be changed after ordering.*
+
 ## How to support
 
 [Support cpp-linter on Open Collective](https://opencollective.com/cpp-linter){ .md-button .md-button--primary }