docs: Update plan mode restrictions (anomalyco#8290)

Author: Eduard-Voiculescu

packages/web/src/content/docs/modes.mdx

@@ -34,7 +34,7 @@ Build is the **default** mode with all tools enabled. This is the standard mode
 A restricted mode designed for planning and analysis. In plan mode, the following tools are disabled by default:
 
 - `write` - Cannot create new files
-- `edit` - Cannot modify existing files
+- `edit` - Cannot modify existing files, except for files located at `.opencode/plans/*.md` to detail the plan itself
 - `patch` - Cannot apply patches
 - `bash` - Cannot execute shell commands