feat(plan): document and validate Plan Mode policy overrides (#18825)

docs/core/policy-engine.md

@@ -119,9 +119,17 @@ For example:
 
 Approval modes allow the policy engine to apply different sets of rules based on
 the CLI's operational mode. A rule can be associated with one or more modes
-(e.g., `yolo`, `autoEdit`). The rule will only be active if the CLI is running
-in one of its specified modes. If a rule has no modes specified, it is always
-active.
+(e.g., `yolo`, `autoEdit`, `plan`). The rule will only be active if the CLI is
+running in one of its specified modes. If a rule has no modes specified, it is
+always active.
+
+- `default`: The standard interactive mode where most write tools require
+  confirmation.
+- `autoEdit`: Optimized for automated code editing; some write tools may be
+  auto-approved.
+- `plan`: A strict, read-only mode for research and design. See [Customizing
+  Plan Mode Policies].
+- `yolo`: A mode where all tools are auto-approved (use with extreme caution).
 
 ## Rule matching
 
@@ -303,3 +311,5 @@ out-of-the-box experience.
 - In **`yolo`** mode, a high-priority rule allows all tools.
 - In **`autoEdit`** mode, rules allow certain write operations to happen without
   prompting.
+
+[Customizing Plan Mode Policies]: /docs/cli/plan-mode.md#customizing-policies