Envelope budget: Budget page, per-month assignments stored on a Budget tab, rollover and ready-to-assign

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -21,6 +21,12 @@ AppID=money PUID=1000 PGID=1000 TZ=America/New_York docker compose up -d   # the
 "Create a blank workbook" button that copies it into the data folder. Then drop statements in; new payees land on the Lists tab as
 "Other / Unsorted" for you to categorize. `loader_rules.example.json` shows the private rules file format.
 
+## Envelope budget
+The **Budget** page (`/budget`) turns each category into an envelope. Each month, assign your income to envelopes; the goal is
+"Ready to assign" = $0. Spending comes from the Bill Log, leftovers roll over, and overspending carries forward as a negative.
+Tools: *Copy last month*, *Suggest from spending* (3-month average) and *Move money*. Amounts are saved to a `Budget` tab in the workbook
+(one row per month and envelope), with a backup first.
+
 ## Categories
 New payees are categorized by built-in generic merchant rules (`seed_rules.py`); anything unknown lands in "Other / Unsorted".
 Use the **Categorize payees** card in the app (or the Lists tab) to set the rest. Your choices are saved in the workbook.