docs: add filetype detection section to AGENTS.md

AGENTS.md

@@ -50,6 +50,18 @@ scl_lsp/
 └── grammar.js               # Tree-sitter grammar
 ```
 
+## Filetype Detection
+
+The LSP requires the `scl` filetype to be set. Add to your Neovim config (e.g., `ftdetect/scl.vim`):
+
+```vim
+au BufRead,BufNewFile *.scl set filetype=scl
+au BufRead,BufNewFile *.udt set filetype=scl
+au BufRead,BufNewFile *.db set filetype=scl
+```
+
+This enables LSP features (completion, goto definition, hover, etc.) for all SCL-related file types.
+
 ## Code Style Guidelines
 
 ### Module Pattern