fix: stop tracking chat title in audit logs (#24564)

Chat titles can contain sensitive information (secrets, internal project
names, etc.) and should not be visible in audit logs.

- Use truncated chat UUID (first 8 chars) as `resource_target` instead
of the title
- Mark the `title` field as `ActionSecret` so diffs render as `••••••••`

<details><summary>Implementation notes</summary>

Two changes:
1. `coderd/audit/request.go`: `ResourceTarget` for Chat returns
`typed.ID.String()[:8]` instead of `typed.Title`
2. `enterprise/audit/table.go`: Chat `title` field tracking changed from
`ActionTrack` to `ActionSecret`

No frontend changes needed. The frontend already handles `secret: true`
fields.

</details>

> 🤖

coderd/audit/request.go

@@ -135,7 +135,7 @@ func ResourceTarget[T Auditable](tgt T) string {
 	case database.AiSeatState:
 		return "AI Seat"
 	case database.Chat:
-		return typed.Title
+		return typed.ID.String()[:8]
 	default:
 		panic(fmt.Sprintf("unknown resource %T for ResourceTarget", tgt))
 	}