[Tabby] Automatically include related files in chat context

## 💡 Description Currently, when using the Tabby IntelliJ plugin, the quality of completions in the chat or inline suggestions can drop significantly if not all related files are manually added to the chat context. For example: * I'm editing `A.java`, which calls `foo()` from `B.java`. * The method `foo()` returns a `C` class defined in `C.java`. * If I only add `A.java` to the chat, Tabby often hallucinates — such as calling `foo()` with wrong parameters, or accessing non-existent properties in `C`. To avoid this, I have to manually click **“Add file to chat”** for `B.java` and `C.java`. However, this process is manual and error-prone, especially in large projects where dependencies are not obvious. --- ## 🧱 Real-world Use Case In our Java project, a typical service or logic file depends on multiple model, utility, and helper classes across different packages. When writing or modifying a method, I often need related files (e.g., `Entity`, `Mapper`, `DTO`) to be included in co