feat: add database migration v9→v10 for Hermes support (Phase 2)

- Bump SCHEMA_VERSION from 9 to 10
- Add enabled_hermes column to mcp_servers and skills tables
- Add migrate_v9_to_v10 with table_exists guard for skills (may not
  exist in databases migrated from very old versions)
- Update dao/mcp.rs to fully read/write enabled_hermes in all queries
- Update dao/skills.rs: don't SELECT enabled_hermes (Hermes doesn't
  support Skills yet), keep column indices clean

src-tauri/src/database/mod.rs

@@ -44,7 +44,7 @@ use std::sync::Mutex;
 
 /// 当前 Schema 版本号
 /// 每次修改表结构时递增，并在 schema.rs 中添加相应的迁移逻辑
-pub(crate) const SCHEMA_VERSION: i32 = 9;
+pub(crate) const SCHEMA_VERSION: i32 = 10;
 
 /// 安全地序列化 JSON，避免 unwrap panic
 pub(crate) fn to_json_string<T: Serialize>(value: &T) -> Result<String, AppError> {