refactor: migrate troubleshoot-agent from KV to D1

- Create troubleshoot_sessions table in D1
- Replace KV session storage with D1
- Unify field names to snake_case (user_id, collected_info, created_at, updated_at, expires_at)
- Add __PASSTHROUGH__/__SESSION_END__ marker support
- Change handler signature to match domain/deposit pattern
- Extract system prompt to constant TROUBLESHOOT_EXPERT_PROMPT
- Add hasTroubleshootSession() for routing
- Update openai-service.ts to use new D1-based functions
- Update troubleshoot-tool.ts to use D1 instead of KV
- Add TroubleshootSessionStatus type

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

migrations/007_add_troubleshoot_sessions.sql

@@ -0,0 +1,14 @@
+-- Troubleshoot Agent Sessions (D1)
+-- 기존 KV 세션을 D1로 마이그레이션
+
+CREATE TABLE IF NOT EXISTS troubleshoot_sessions (
+  user_id TEXT PRIMARY KEY,
+  status TEXT NOT NULL CHECK(status IN ('gathering', 'diagnosing', 'suggesting', 'completed')),
+  collected_info TEXT,
+  messages TEXT,
+  created_at INTEGER NOT NULL,
+  updated_at INTEGER NOT NULL,
+  expires_at INTEGER NOT NULL
+);
+
+CREATE INDEX IF NOT EXISTS idx_troubleshoot_sessions_expires ON troubleshoot_sessions(expires_at);