Add audit subagent and restructure skill for progressive disclosure

- Add audit subagent that runs automatically after bill entries
- Create download-attachment.sh for retrieving invoice/receipt PDFs
- Create verify-pdf.py for PDF extraction with OCR fallback
- Restructure SKILL.md from 856 to 177 lines using reference files
- Move detailed content to references/:
  - schema.md: table schemas
  - workflows.md: code examples
  - queries.md: SQL queries and financial reports
  - audit.md: audit queries and remediation steps

references/schema.md

@@ -0,0 +1,97 @@
+# Database Schema
+
+Complete table schemas for the Grist accounting system.
+
+## Accounts
+| Column | Type | Notes |
+|--------|------|-------|
+| Code | Text | Account number (e.g., "5080") |
+| Name | Text | Account name |
+| Type | Choice | "Asset", "Liability", "Equity", "Income", "Expense" |
+| Parent | Ref:Accounts | Parent account for hierarchy (0 = top-level) |
+| Description | Text | |
+| IsActive | Bool | |
+| Balance | Formula | Calculated from transactions |
+
+## Vendors
+| Column | Type | Notes |
+|--------|------|-------|
+| Name | Text | Vendor name |
+| DefaultExpenseAccount | Ref:Accounts | Auto-fills on bill lines |
+| PaymentTerms | Choice | "Due on Receipt", "Net 15", "Net 30" |
+| Notes | Text | |
+| IsActive | Bool | |
+| Balance | Formula | Sum of unpaid bills |
+
+## Items
+| Column | Type | Notes |
+|--------|------|-------|
+| Name | Text | Item name (e.g., "Software Subscription") |
+| DefaultAccount | Ref:Accounts | Expense account for this item |
+| DefaultDescription | Text | Auto-fills on bill lines |
+| IsActive | Bool | |
+
+## Bills
+| Column | Type | Notes |
+|--------|------|-------|
+| Vendor | Ref:Vendors | Required |
+| BillNumber | Text | Invoice number from vendor |
+| BillDate | Date | Unix timestamp |
+| DueDate | Date | Unix timestamp |
+| Status | Choice | "Open", "Partial", "Paid" |
+| Memo | Text | |
+| EntryTransaction | Ref:Transactions | Link to journal entry |
+| Invoice | Attachments | Vendor invoice document (use `["L", id]` format) |
+| Receipt | Attachments | Payment receipt/confirmation (use `["L", id]` format) |
+| Amount | Formula | Sum of BillLines.Amount |
+| AmountPaid | Formula | Sum of BillPayments.Amount |
+| AmountDue | Formula | Amount - AmountPaid |
+
+## BillLines
+| Column | Type | Notes |
+|--------|------|-------|
+| Bill | Ref:Bills | Required |
+| Item | Ref:Items | Optional - auto-fills Account/Description |
+| Account | Ref:Accounts | Expense account |
+| Description | Text | |
+| Amount | Numeric | Line item amount |
+
+## Transactions
+| Column | Type | Notes |
+|--------|------|-------|
+| Date | Date | Unix timestamp |
+| Description | Text | Transaction description |
+| Reference | Text | Check number, invoice reference, etc. |
+| Status | Choice | "Draft", "Posted", "Cleared" |
+| Memo | Text | |
+| Total | Formula | Sum of debits |
+| IsBalanced | Formula | True if debits = credits |
+
+## TransactionLines
+| Column | Type | Notes |
+|--------|------|-------|
+| Transaction | Ref:Transactions | Required |
+| Account | Ref:Accounts | Required |
+| Debit | Numeric | Debit amount (or 0) |
+| Credit | Numeric | Credit amount (or 0) |
+| Memo | Text | |
+
+## BillPayments
+| Column | Type | Notes |
+|--------|------|-------|
+| Bill | Ref:Bills | Required |
+| Transaction | Ref:Transactions | Payment journal entry |
+| Amount | Numeric | Payment amount |
+| PaymentDate | Date | Unix timestamp |
+
+## Formula Columns (Auto-Calculated)
+
+| Table.Column | Description |
+|--------------|-------------|
+| Accounts.Balance | OwnBalance + ChildrenBalance |
+| Transactions.IsBalanced | True if sum of debits = sum of credits |
+| Transactions.Total | Sum of debit amounts |
+| Bills.Amount | Sum of BillLines.Amount |
+| Bills.AmountPaid | Sum of BillPayments.Amount |
+| Bills.AmountDue | Amount - AmountPaid |
+| Vendors.Balance | Sum of AmountDue for unpaid bills |