chore: load auth from .env file

README.md

@@ -4,6 +4,16 @@ My solutions to the [advent of code](https://adventofcode.com/) challenges, writ
 
 ## How to run
 
+### Setup
+
+To use, you need a session token from Advent of Code:
+
+1. Log in at https://adventofcode.com
+2. Copy your session cookie (check browser dev tools)
+3. Either:
+   - Set `AOC_SESSION` environment variable, or
+   - Create a `.env` file in project root with: `AOC_SESSION=your_token`
+
 ### Install project
 
 Run `make install` or