Rewrite `os.environ.get` as `os.getenv`

**Bad**

```py
os.environ.get(secret)
os.environ.get(secret, "fallback")
```

**Good**

```py
os.getenv(secret)
os.getenv(secret, "fallback")
```

Start by locating Ruff's implementation and tests for rules that rewrite Python expressions involving os.environ. Verify how the rule recognizes both one-argument and fallback forms shown in the issue, and confirm that the resulting code uses os.getenv with equivalent arguments.

Written by the indexing model from the issue text.