Michaelb/add end date cli option

[mbostler]

This PR adds the ability to parse and "end-date" CLI option to restrict the returned metrics to a specific time window.

This was found to be necessary for Q1 26 QBR data collection in order to avoid running into timeout errors when running the script.

This new filter can be activated by passing -e or --end-date option, eg:

./fetch-code-review-metrics.py --repo stepful/stepful --file ./metrics_2026-01-14-full.csv --token ghp_XXX --start-date 2025-10-01 --end-date 2025-10-31

[dangoslen]

One question about the script, but the rest looks good to me

[dangoslen]

Was that meant to be committed? It is great, but seems like a custom script?

Maybe we could put scripts like this in a /useful-scripts or /analysis directory?