refactor: move deposit-agent to agents/ and add session support

- Move deposit-agent.ts to src/agents/
- Add D1 session CRUD functions (getDepositSession, saveDepositSession, etc.)
- Add Deposit Expert AI with function calling
- Add processDepositConsultation handler for session-based flow
- Add deposit_sessions table migration (006_add_deposit_sessions.sql)
- Update import paths in deposit-tool.ts
- Add DEPOSIT_BANK_* environment variables to Env interface

Session flow: collecting_amount → collecting_name → confirming → completed
Smart parsing: "홍길동 5만원" → Go directly to confirming

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

migrations/006_add_deposit_sessions.sql

@@ -0,0 +1,16 @@
+-- Migration: Add deposit_sessions table for session-based deposit requests
+-- Date: 2026-02-05
+-- Description: 입금 신고 세션 관리 (collecting_amount → collecting_name → confirming → completed)
+
+CREATE TABLE IF NOT EXISTS deposit_sessions (
+  user_id TEXT PRIMARY KEY,
+  status TEXT NOT NULL CHECK(status IN ('collecting_amount', 'collecting_name', 'confirming', 'completed')),
+  collected_info TEXT, -- JSON: { amount?: number, depositor_name?: string }
+  messages TEXT, -- JSON: [{ role: 'user' | 'assistant', content: string }]
+  created_at INTEGER NOT NULL,
+  updated_at INTEGER NOT NULL,
+  expires_at INTEGER NOT NULL
+);
+
+-- Index for cleanup queries (expired sessions)
+CREATE INDEX IF NOT EXISTS idx_deposit_sessions_expires_at ON deposit_sessions(expires_at);