feat(coderd): add task prompt modification endpoint (#20811)

This PR adds the backend implementation for modifying task prompts. Part
of https://github.com/coder/internal/issues/1084

## Changes

- New `UpdateTaskPrompt` database query to update task prompts
- New PATCH `/api/v2/tasks/{task}/prompt` endpoint

## Notes

This is part 1 of a 2-part PR stack. The frontend UI will be added in a
follow-up PR based on this branch
(https://github.com/coder/coder/pull/20812).

---

🤖 PR was written by Claude Sonnet 4.5 Thinking using [Coder
Mux](https://github.com/coder/cmux) and reviewed by a human 👩

coderd/database/queries/tasks.sql

@@ -64,3 +64,14 @@ WHERE
 	id = @id::uuid
 	AND deleted_at IS NULL
 RETURNING *;
+
+
+-- name: UpdateTaskPrompt :one
+UPDATE
+	tasks
+SET
+	prompt = @prompt::text
+WHERE
+	id = @id::uuid
+	AND deleted_at IS NULL
+RETURNING *;