docs(plan): document hooks with plan mode (#22197)

docs/cli/plan-mode.md

@@ -109,16 +109,6 @@ switch back to another mode.
 - **Keyboard shortcut:** Press `Shift+Tab` to cycle to the desired mode.
 - **Natural language:** Ask Gemini CLI to "exit plan mode" or "stop planning."
 
-## Customization and best practices
-
-Plan Mode is secure by default, but you can adapt it to fit your specific
-workflows. You can customize how Gemini CLI plans by using skills, adjusting
-safety policies, or changing where plans are stored.
-
-## Commands
-
-- **`/plan copy`**: Copy the currently approved plan to your clipboard.
-
 ## Tool Restrictions
 
 Plan Mode enforces strict safety policies to prevent accidental changes.
@@ -146,6 +136,12 @@ These are the only allowed tools:
 - **Skills:** [`activate_skill`](../cli/skills.md) (allows loading specialized
   instructions and resources in a read-only manner)
 
+## Customization and best practices
+
+Plan Mode is secure by default, but you can adapt it to fit your specific
+workflows. You can customize how Gemini CLI plans by using skills, adjusting
+safety policies, changing where plans are stored, or adding hooks.
+
 ### Custom planning with skills
 
 You can use [Agent Skills](../cli/skills.md) to customize how Gemini CLI
@@ -294,6 +290,71 @@ modes = ["plan"]
 argsPattern = "\"file_path\":\"[^\"]+[\\\\/]+\\.gemini[\\\\/]+plans[\\\\/]+[\\w-]+\\.md\""
 ```
 
+### Using hooks with Plan Mode
+
+You can use the [hook system](../hooks/writing-hooks.md) to automate parts of
+the planning workflow or enforce additional checks when Gemini CLI transitions
+into or out of Plan Mode.
+
+Hooks such as `BeforeTool` or `AfterTool` can be configured to intercept the
+`enter_plan_mode` and `exit_plan_mode` tool calls.
+
+> [!WARNING] When hooks are triggered by **tool executions**, they do **not**
+> run when you manually toggle Plan Mode using the `/plan` command or the
+> `Shift+Tab` keyboard shortcut. If you need hooks to execute on mode changes,
+> ensure the transition is initiated by the agent (e.g., by asking "start a plan
+> for...").
+
+#### Example: Archive approved plans to GCS (`AfterTool`)
+
+If your organizational policy requires a record of all execution plans, you can
+use an `AfterTool` hook to securely copy the plan artifact to Google Cloud
+Storage whenever Gemini CLI exits Plan Mode to start the implementation.
+
+**`.gemini/hooks/archive-plan.sh`:**
+
+```bash
+#!/usr/bin/env bash
+# Extract the plan path from the tool input JSON
+plan_path=$(jq -r '.tool_input.plan_path // empty')
+
+if [ -f "$plan_path" ]; then
+  # Generate a unique filename using a timestamp
+  filename="$(date +%s)_$(basename "$plan_path")"
+
+  # Upload the plan to GCS in the background so it doesn't block the CLI
+  gsutil cp "$plan_path" "gs://my-audit-bucket/gemini-plans/$filename" > /dev/null 2>&1 &
+fi
+
+# AfterTool hooks should generally allow the flow to continue
+echo '{"decision": "allow"}'
+```
+
+To register this `AfterTool` hook, add it to your `settings.json`:
+
+```json
+{
+  "hooks": {
+    "AfterTool": [
+      {
+        "matcher": "exit_plan_mode",
+        "hooks": [
+          {
+            "name": "archive-plan",
+            "type": "command",
+            "command": "./.gemini/hooks/archive-plan.sh"
+          }
+        ]
+      }
+    ]
+  }
+}
+```
+
+## Commands
+
+- **`/plan copy`**: Copy the currently approved plan to your clipboard.
+
 ## Planning workflows
 
 Plan Mode provides building blocks for structured research and design. These are