fix: improve task prompt placeholder consistency (coder#18189)

Updates the placeholder text in the task prompt box to be consistent
with the "task" terminology used throughout the UI.

**Changes:**
- Changed placeholder from "Write an action for your AI agent to
perform..." to "Prompt your AI agent to start a task..."
- This aligns with the "Run task" button text and overall task-focused
language

**Testing:**
- Verified the text change renders correctly in the UI
- No functional changes, only text update

Co-authored-by: blink-so <blink-so@users.noreply.github.com>

Author: blink-so[bot]

site/src/pages/TasksPage/TasksPage.tsx

@@ -209,7 +209,7 @@ const TaskForm: FC<TaskFormProps> = ({ templates }) => {
 					required
 					id="prompt"
 					name="prompt"
-					placeholder="Write an action for your AI agent to perform..."
+					placeholder="Prompt your AI agent to start a task..."
 					className={`border-0 resize-none w-full h-full bg-transparent rounded-lg outline-none flex min-h-[60px]
 						text-sm shadow-sm text-content-primary placeholder:text-content-secondary md:text-sm`}
 				/>