fix(agents): persist system prompt server-side instead of localStorage (#22857)

## Problem

The Admin → Agents → System Prompt textarea saved only to the browser's
`localStorage`. The value was never sent to the backend, never stored in
the database, and never injected into chats. Entering text, clicking
Save, and refreshing the page showed no changes — the prompt was
effectively a no-op.

## Root Cause

Three disconnected layers:
1. **Frontend** wrote to `localStorage`, never called an API.
2. **`handleCreateChat`** never read `savedSystemPrompt`.
3. **Backend** hardcoded `chatd.DefaultSystemPrompt` on every chat
creation — no field in `CreateChatRequest` accepted a custom prompt.

## Changes

### Database
- Added `GetChatSystemPrompt` / `UpsertChatSystemPrompt` queries on the
existing `site_configs` table (no migration needed).

### API
- `GET /api/experimental/chats/system-prompt` — returns the configured
prompt (any authenticated user).
- `PUT /api/experimental/chats/system-prompt` — sets the prompt
(admin-only, `rbac: deployment_config update`).
- Input validation: max 32 KiB prompt length.

### Backend
- `resolvedChatSystemPrompt(ctx)` checks for a custom prompt in the DB,
falls back to `chatd.DefaultSystemPrompt` when empty/unset.
- Logs a warning on DB errors instead of silently swallowing them.
- Replaced the hardcoded `defaultChatSystemPrompt()` call in chat
creation.

### Frontend
- Replaced `localStorage` read/write with React Query
`useQuery`/`useMutation` backed by the new endpoints.
- Fixed `useEffect` draft sync to avoid clobbering in-progress user
edits on refetch.
- Added `try/catch` error handling on save (draft stays dirty for
retry).
- Save button disabled during mutation (`isSavingSystemPrompt`).
- Query key follows kebab-case convention (`chat-system-prompt`).

### UX
- Added hint: "When empty, the built-in default prompt is used."

### Tests
- `TestChatSystemPrompt`: GET returns empty when unset, admin can set,
non-admin gets 403.
- dbauthz `TestMethodTestSuite` coverage for both new querier methods.

coderd/coderd.go

@@ -1127,6 +1127,11 @@ func New(options *Options) *API {
 				r.Post("/", api.postChatFile)
 				r.Get("/{file}", api.chatFileByID)
 			})
+			r.Route("/config", func(r chi.Router) {
+				r.Get("/system-prompt", api.getChatSystemPrompt)
+				r.Put("/system-prompt", api.putChatSystemPrompt)
+			})
+			// TODO(cian): place under /api/experimental/chats/config
 			r.Route("/providers", func(r chi.Router) {
 				r.Get("/", api.listChatProviders)
 				r.Post("/", api.createChatProvider)
@@ -1135,6 +1140,7 @@ func New(options *Options) *API {
 					r.Delete("/", api.deleteChatProvider)
 				})
 			})
+			// TODO(cian): place under /api/experimental/chats/config
 			r.Route("/model-configs", func(r chi.Router) {
 				r.Get("/", api.listChatModelConfigs)
 				r.Post("/", api.createChatModelConfig)