docs: add note that agent-mode-switcher.json is auto-generated (#24)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: j4rviscmd

README.md

@@ -53,7 +53,11 @@ The following command files are automatically copied to
 ### Recommended: Edit Configuration File Directly
 
 The recommended way to switch modes is to edit
-`~/.config/opencode/agent-mode-switcher.json` and change the `currentMode` value:
+`~/.config/opencode/agent-mode-switcher.json` and change the `currentMode`
+value:
+
+> [!NOTE]
+> This file is automatically generated on first plugin startup.
 
 ```json
 {