Diffs: restore system prompt guidance (#36904)

Merged via squash.

Prepared head SHA: 1b3be3c879
Co-authored-by: gumadeiras <5599352+gumadeiras@users.noreply.github.com>
Co-authored-by: gumadeiras <5599352+gumadeiras@users.noreply.github.com>
Reviewed-by: @gumadeiras

docs/tools/diffs.md

@@ -10,7 +10,7 @@ read_when:
 
 # Diffs
 
-`diffs` is an optional plugin tool and companion skill that turns change content into a read-only diff artifact for agents.
+`diffs` is an optional plugin tool with short built-in system guidance and a companion skill that turns change content into a read-only diff artifact for agents.
 
 It accepts either:
 
@@ -23,6 +23,8 @@ It can return:
 - a rendered file path (PNG or PDF) for message delivery
 - both outputs in one call
 
+When enabled, the plugin prepends concise usage guidance into system-prompt space and also exposes a detailed skill for cases where the agent needs fuller instructions.
+
 ## Quick start
 
 1. Enable the plugin.
@@ -44,6 +46,29 @@ It can return:
 }
 ```
 
+## Disable built-in system guidance
+
+If you want to keep the `diffs` tool enabled but disable its built-in system-prompt guidance, set `plugins.entries.diffs.hooks.allowPromptInjection` to `false`:
+
+```json5
+{
+  plugins: {
+    entries: {
+      diffs: {
+        enabled: true,
+        hooks: {
+          allowPromptInjection: false,
+        },
+      },
+    },
+  },
+}
+```
+
+This blocks the diffs plugin's `before_prompt_build` hook while keeping the plugin, tool, and companion skill available.
+
+If you want to disable both the guidance and the tool, disable the plugin instead.
+
 ## Typical agent workflow
 
 1. Agent calls `diffs`.