chat: rename the pi-mono provider router + purge residual pi names (Stage 4b/2)

Renames the AI-harness/provider settings router into the chat namespace and
clears the remaining "pi" identifiers from the chat stack.

- server-settings/pi-mono.ts → chat-providers.ts; piMonoRouter → chatProvidersRouter;
  route /server-settings/pi-mono → /server-settings/chat-providers (+ all callers)
- piId → providerId (PROVIDERS map + AIHarnessesSection UI), PiProvider → ChatProvider,
  PI_MONO_* query keys → CHAT_PROVIDERS_*, installPiMono → installAgent
- data-path: PI_CONFIG_DIR → AGENT_CONFIG_DIR (path ~/.pi/agent unchanged);
  drop dead getPiMonoDir/getPiMonoSessionDir exports
- settings: flip the vestigial defaultProvider literal 'pi' → 'chat' (never read;
  only defaultModel drives behavior); access-policy config key 'pi-access-policy'
  → 'chat-access-policy'
- misc: ModelSelector fallback label, TaskDefaults model grouping, CapabilityPage
  chat var, a stale stream-parser comment

Intentionally left (genuine external `pi`/opencode references, not ours to rename):
the `pi` binary install/version flow (@mariozechner/pi-coding-agent, `which pi`),
the ~/.pi/agent config path, PI_TOOLS_DIRS/PI_SEARXNG_URL runtime env-var contract,
TOOL.md `targets: pi` metadata, and the "Pi Mono" installer UI label.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/apps/officer-web/Screens/Dashboard/CapabilityPage.tsx

@@ -279,20 +279,20 @@ export const CapabilityChat = ({
     ? (description ?? `Help me create the content for this new ${kind} file`)
     : `Help me understand and improve this ${kind} file`;
 
-  const pi = useChat(undefined, undefined, { replaceUrl: false });
+  const chat = useChat(undefined, undefined, { replaceUrl: false });
 
   const onResponseEndRef = useRef(onResponseEnd);
   onResponseEndRef.current = onResponseEnd;
 
   const wasGenerating = useRef(false);
   useEffect(() => {
-    if (wasGenerating.current && !pi.isGenerating) {
+    if (wasGenerating.current && !chat.isGenerating) {
       onResponseEndRef.current?.();
     }
-    wasGenerating.current = pi.isGenerating;
-  }, [pi.isGenerating]);
+    wasGenerating.current = chat.isGenerating;
+  }, [chat.isGenerating]);
 
-  return <EmbeddableChat chat={pi} defaultInput={defaultInput} promptPrefix={promptFrontmatter} className="h-full" />;
+  return <EmbeddableChat chat={chat} defaultInput={defaultInput} promptPrefix={promptFrontmatter} className="h-full" />;
 };
 
 export const FrontmatterBlock = ({ yaml }: { yaml: string }) => {