docs: add IntelliJ LSP4IJ setup + Azure Toolkit conflict warning

When Azure Toolkit is also installed, IDEA merges its native Bicep
completions with LSP4IJ completions — causing resource/projectName/schema
noise that bypasses the iLSP proxy entirely.

EDITOR_SETUP.md

@@ -12,6 +12,35 @@ supports LSP WebSocket transport.
 
 ---
 
+## IntelliJ IDEA setup (LSP4IJ)
+
+Install the [LSP4IJ](https://plugins.jetbrains.com/plugin/23257-lsp4ij) plugin, then:
+
+1. **Settings → Languages & Frameworks → LSP → Language Servers → +**
+   - Name: `iLSP Bicep`
+   - Server type: `WebSocket`
+   - URL: `wss://ilsp.i80.dk/bicep`
+
+2. **Add a file-type mapping** under the new server entry:
+   - File type: `Bicep` (or pattern `*.bicep`)
+
+3. **Disable Azure Toolkit Bicep completions** — this is critical if you have the
+   Azure Toolkit plugin installed. Without this step, IDEA merges completions from
+   both LSP4IJ and Azure Toolkit, resulting in noisy suggestions (`resource`,
+   `projectName`, Bicep schema types, etc.) appearing alongside iLSP results.
+
+   Go to **Settings → Languages & Frameworks → Azure Toolkit for IntelliJ**
+   and disable or uncheck Bicep language support. If there is no such option,
+   disable the Azure Toolkit plugin entirely for Bicep projects, or suppress
+   its completion contributor via **Settings → Editor → General → Code Completion**
+   (uncheck "Show suggestions from plugins that don't support the current language server").
+
+   > **Why**: LSP4IJ feeds completions through the LSP protocol where iLSP can
+   > filter them. Azure Toolkit injects completions directly into IDEA's completion
+   > system, bypassing the LSP layer entirely — iLSP cannot suppress those.
+
+---
+
 ## Neovim setup
 
 ```lua