Append install-wide instructions to every bot's prompt

Settings > General gains a prompt appendix the super admin sets once:
markdown every bot runs with, right after its own core instructions.

- The text lives in app_settings (bot_system_prompt_append, read through
  grabSystemPromptAppend, cached like every app setting), so it changes
  without a deploy and applies to every bot on the next run.
- composeBotPrompt takes an appendix and pushes it as
  # Install-wide instructions directly after system_prompt, before the
  workspace block and memory, so the bot's own more specific rules still
  win. grabBotPrompt feeds it in, so chat replies, tasks, scheduled runs
  and the coordinator all get it with no call site touched.
- api/admin/settings/system-prompt-append saves it (super admin only,
  capped at AppData.PromptMaxLength, empty clears it) and is audit
  logged under system.
- The section and its form are hidden from everyone but the super admin,
  save in place with a character count and no reload, and say what an
  empty text does.

src/pages/admin/settings/general/index.tsx

@@ -1,4 +1,5 @@
 import AppearanceSection from "./(sections)/appearance-section";
+import PromptSection from "./(sections)/prompt-section";
 import StorageSection from "./(sections)/storage-section";
 import SettingsShell from "../(partials)/settings-shell";
 import settingsPageMeta from "../(functions)/settings-page-meta";
@@ -10,6 +11,7 @@ export default function AdminSettingsGeneralPage() {
             description="How the app looks and behaves."
         >
             <AppearanceSection />
+            <PromptSection />
             <StorageSection />
         </SettingsShell>
     );