Support for "paid by partner" tag.

Added support for a configurable transaction tag (PAID_BY_PARTNER_TAG, default "Paid by partner") and loaded it from environment config, so the behavior can be customized without code changes.

Extended transaction modeling to include tags and added a helper that detects a matching tag whether tags come as plain strings or objects with a name field.

Updated bill computation so the Jira “total expenses” still reflects all included shared expenses, while the owed half is adjusted using your requested rule (half = total/2 - paid_by_other_total). Also tracked count/amount of tagged transactions in BillingResult.

Updated Jira description/log output to include the fully-paid-by-other adjustment details for transparency in each monthly task.

Added/updated tests and assertions for the new result fields.

Documented the new feature and environment variable in the README (features, calculation flow, and optional config table).