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/coderd.go

@@ -1036,6 +1036,7 @@ func New(options *Options) *API {
 					r.Use(httpmw.ExtractTaskParam(options.Database))
 					r.Get("/", api.taskGet)
 					r.Delete("/", api.taskDelete)
+					r.Patch("/input", api.taskUpdateInput)
 					r.Post("/send", api.taskSend)
 					r.Get("/logs", api.taskLogs)
 				})