Document the API key rotate endpoint (#413)

Replace the manual create-then-delete rotation steps with the rotate
endpoint, covering days_to_expire, the expire_in_days grace window, and the
related troubleshooting cases.

Author: masnwilliams

info/api-keys.mdx

@@ -212,18 +212,85 @@ func main() {
 
 ## Rotate a key
 
-Rotate by creating the replacement first, then deleting the old key after your workload has switched over.
+`rotate` issues a replacement key in a single call and keeps the old key working for a short grace period, so your workload can switch over without downtime. The new key copies the rotated key's name and project scope, and—like create—Kernel returns the plaintext `key` only once.
 
-1. Create a new API key with the same scope.
-2. Store the new plaintext key in your secret manager.
-3. Deploy or restart the workload that uses `KERNEL_API_KEY`.
-4. Verify the workload can call Kernel.
-5. Delete the old API key.
+Two optional parameters control the timing:
+
+- `days_to_expire` sets the new key's lifetime in days (`1`-`3650`). Omit it to give the new key the same lifetime the rotated key originally had, or to never expire if the old key never did.
+- `expire_in_days` sets how long the old key keeps working before it expires. Use `0` to revoke it immediately, or omit it for the default 7-day grace window. The old key stops authenticating automatically once the window passes—you don't need to delete it.
+
+<CodeGroup>
+```typescript TypeScript
+const rotated = await kernel.apiKeys.rotate('key_01jwv4tn5m8k3q2v7x9p0a1bc2', {
+  days_to_expire: 30,
+  expire_in_days: 7,
+});
+
+console.log(rotated.key); // Save this value now. Kernel won't show it again.
+console.log(rotated.id, rotated.masked_key);
+```
+
+```python Python
+import os
+from kernel import Kernel
+
+client = Kernel(api_key=os.environ["KERNEL_API_KEY"])
+
+rotated = client.api_keys.rotate(
+    "key_01jwv4tn5m8k3q2v7x9p0a1bc2",
+    days_to_expire=30,
+    expire_in_days=7,
+)
+
+print(rotated.key)  # Save this value now. Kernel won't show it again.
+print(rotated.id, rotated.masked_key)
+```
+
+```go Go
+package main
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/kernel/kernel-go-sdk"
+)
+
+func main() {
+	ctx := context.Background()
+	client := kernel.NewClient()
+
+	rotated, err := client.APIKeys.Rotate(
+		ctx,
+		"key_01jwv4tn5m8k3q2v7x9p0a1bc2",
+		kernel.APIKeyRotateParams{
+			DaysToExpire: kernel.Int(30),
+			ExpireInDays: kernel.Int(7),
+		},
+	)
+	if err != nil {
+		panic(err)
+	}
+
+	fmt.Println(rotated.Key) // Save this value now. Kernel won't show it again.
+	fmt.Println(rotated.ID, rotated.MaskedKey)
+}
+```
+</CodeGroup>
+
+After you rotate a key:
+
+1. Store the new plaintext key in your secret manager.
+2. Deploy or restart the workload that uses `KERNEL_API_KEY`.
+3. Verify the workload can call Kernel before the grace window ends.
+
+To cut over immediately instead of using a grace window, pass `expire_in_days: 0` so the old key stops working as soon as the new one is issued.
 
 ## Troubleshooting
 
 | Error | What it means | What to do |
 | --- | --- | --- |
-| `400 Bad Request` | The name is missing, `days_to_expire` is outside `1`-`3650`, or `project_id` is empty. | Send a name, choose a valid expiry, or omit `project_id` for an org-scoped key. |
+| `400 Bad Request` | The name is missing, `days_to_expire` is outside `1`-`3650`, `expire_in_days` is outside `0`-`3650`, or `project_id` is empty. | Send a name, choose a valid expiry, or omit `project_id` for an org-scoped key. |
+| `400 Bad Request` (rotate) | `days_to_expire` is shorter than `expire_in_days`, so the new key would expire before the old key's grace window ends. | Raise `days_to_expire` or lower `expire_in_days`. |
 | `401 Unauthorized` | Kernel couldn't authenticate the request. | Set a valid `KERNEL_API_KEY`. |
-| `404 Not Found` | The project doesn't exist or the caller can't access it. | Check the project ID. If you're using a project-scoped key, create keys only for that same project. |
+| `404 Not Found` | The project or API key doesn't exist, or the caller can't access it. | Check the ID. If you're using a project-scoped key, you can only rotate keys in that same project. |