docs: add commit message conventions with 50/72 rule

AGENTS.md

@@ -63,6 +63,23 @@ to the custom scripts instead of system-installed packages.
   - Windows/PowerShell: use `` ` `` for line continuation
   - Unix/Linux/macOS: use `\` for line continuation
 
+## Commit Messages
+
+- Follow the 50/72 rule:
+  - Subject line: max 50 characters
+  - Body lines: wrapped at 72 characters
+- Use conventional commit prefixes (`feat:`, `fix:`, `docs:`, `chore:`, etc.)
+- Separate subject from body with a blank line
+
+Example:
+
+  ```
+  feat: add stopwatch timer
+
+  Replace Hello World with a live stopwatch that prints elapsed time
+  in HH:MM:SS.mmm format, updating every 10ms with color output.
+  ```
+
 ## Source Layout
 
 ```