feat: tax categories and the tax summary report

wanderbooks is a general MCP-first accounting application: every
business needs its accounts mapped to where they land on its income tax
return, and its CPA needs book-to-tax differences surfaced, not hidden.

- ledger_core::tax: a catalog of stable categories (gross_receipts,
  commissions_fees, meals, guaranteed_payments, officer_compensation,
  nondeductible, ...) with kind (income, cost of goods, deduction), the
  returns each appears on (Schedule C, 1065, 1120-S, 1120), a default
  deductible percent (meals 50, nondeductible 0), and guidance. Keys,
  not line numbers: form lines move from year to year.
- Accounts carry tax_category and an optional deductible_percent
  override, validated against the account type (and the return, at the
  MCP layer), journaled as account.tax_set, replayed, projected (schema
  v6).
- tax_summary(start, end, form): the P&L grouped by category, book and
  deductible amounts (half away from zero to the cent), contra income
  (refunds) reducing receipts, and every account still uncategorized.
- MCP: books.tax_categories (filtered to the configured return),
  books.set_tax_category, a tax_summary report in every format and in
  the CPA package, chart output with categories, and a fifth
  tax_category column in chart CSV imports, validated before anything is
  created. [company] tax_form, or derived from tax_classification.

Scope stays book basis: no tax depreciation, basis, or filing.

Tests: a hand-computed quarter (50% meals 333.33 -> 166.67, penalties at
0%, refunds as negative receipts, guaranteed payments, one uncategorized
account; book net 5,276.67 vs tax 5,483.33); overrides; category/account
/return validation; form parsing and derivation; CSV parsing; stdio:
filtered catalog, mapping, refusal, summary, all-or-nothing import.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MtYEaR6M6Ybdhm6X98BWNu

docs/mcp-interface.md

@@ -218,6 +218,25 @@ posting.
     get "Members' equity" with members named; the balance sheet separates
     retained earnings from the fiscal year's net income (loss).
 
+## Tools — tax categories
+
+- `books.tax_categories {}` → `{ form?, categories: [{ key, label, kind,
+  default_deductible_percent, note }] }`: where an account can land on the
+  income tax return, filtered to the business's return when known
+  (`[company] tax_form`, or derived from `tax_classification`). Stable
+  semantic keys (`meals`, `guaranteed_payments`, `commissions_fees`, ...),
+  never form line numbers, which change by year.
+- `books.set_tax_category { account, category?, deductible_percent? }`:
+  map a revenue or expense account to a category (null clears),
+  optionally overriding its deductible percent. Journaled as
+  `account.tax_set`.
+- `books.export { report: "tax_summary" }`: book vs. tax amounts by
+  category, the difference where they disagree (meals 50%, penalties 0%),
+  and every P&L account still uncategorized. In the CPA package for the
+  fiscal year to date.
+- `books.import_chart_csv` accepts an optional fifth `tax_category` column,
+  validated before anything is created.
+
 ## Tools — import
 
 - `books.import_bank_csv { csv_text, bank_account_code, default_offset_code,