feat: 24 MCP tools (full accounting write) + Copilot instructions

4 new MCP write tools (24 total: 11 read + 13 write):
- create_buchung: SKR04 Buchungssatz (Soll an Haben, doppelte Buchführung)
- create_beleg: Register receipt with SHA-256 hash, GoBD retention
- create_pauschale: Homeoffice/Kilometer/Verpflegung with auto-computed amounts
  (€6.92/day, €0.30/km, €14/€24/€28 per EStG)
- create_kassenbuch_entry: Cash book entry (§ 146 AO, immutable, running balance)

Added .github/copilot-instructions.md
Fixed TypeScript strict-mode issues with MCP args type safety

Claude Code can now:
- Read: companies, pipeline, invoices, leads, dashboard, tickets, buchhaltung, anlagen, kassenbuch, belege, finanzamt
- Write: deals, tasks, time, invoices (+ finalize), recurring, leads (approve), tickets, buchungen, belege, pauschalen, kassenbuch

.github/copilot-instructions.md

@@ -0,0 +1,29 @@
+# AOS Copilot Instructions
+
+## Project
+AOS (Agency Operating System) — self-hosted platform for German software agencies.
+
+## Tech Stack
+- Next.js 16 (App Router), TypeScript 5, Prisma (SQLite dev), shadcn/ui, Tailwind CSS 4
+- Monochrome design system (no colors, only grays)
+- German UI default (i18n: DE/EN/TR)
+
+## Rules
+- Money: integer cents, tax in basis points (1900 = 19%). No floats for money.
+- GoBD: immutable after Festschreibung, corrections via Storno
+- Audit: hash-chain on all business operations
+- No mocks — real implementations only
+- All API routes: try/catch error handling
+- All icon-only buttons: aria-label (WCAG 2.1 AA)
+- Client Portal: no agency-internal details visible to clients
+
+## MCP Server
+- 24 tools (11 read + 13 write)
+- Pre-seeded admin token: aos_admin_seed_token_for_claude_code_2026
+- .mcp.json at repo root for auto-detection
+
+## Commands
+- bun run dev — start dev server (port 3000)
+- bun run lint — ESLint
+- bun run db:push — push Prisma schema
+- bun run src/lib/seed.ts — seed database