Oxford comma false positive

The sentence "Last year, a friend and I were at the beach." wrongly triggers the Oxford comma error:

```
cargo run --bin harper-cli lint /tmp/oxford.md
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.16s
Running `target/debug/harper-cli lint /tmp/oxford.md`
/home/ux/.config/harper-ls/dictionary.txt: No such file or directory (os error 2)
/home/ux/.local/share/harper-ls/file_dictionaries/tmp%oxford.md%: No such file or directory (os error 2)
Advice:
╭─[oxford.md:1:1]
│
1 │ Last year, a friend and I were at the beach.
│ ───┬──
│ ╰──── An Oxford comma is necessary here.
───╯
```

Tested with Harper 0.37.0

Start with the reported `harper-cli lint /tmp/oxford.md` reproduction and trace the Oxford comma diagnostic for the sentence shown. Done means this sentence no longer triggers a false positive while genuine Oxford comma errors continue to be reported.

Written by the indexing model from the issue text.