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/workspaces/state/src/useAccessPolicy.ts

@@ -17,14 +17,14 @@ export function useAccessPolicy() {
   const { data: policy = { allowedModels: [] } } = useQuery<AccessPolicy>({
     queryKey: QUERY_KEY,
     enabled: isAuthenticated,
-    queryFn: () => client.get<AccessPolicy>('/server-settings/pi-mono/access-policy'),
+    queryFn: () => client.get<AccessPolicy>('/server-settings/chat-providers/access-policy'),
     staleTime: 5 * 60 * 1000,
   });
 
   const savePolicy = useCallback(
     async (newPolicy: AccessPolicy) => {
       queryClient.setQueryData(QUERY_KEY, newPolicy);
-      await client.put('/server-settings/pi-mono/access-policy', newPolicy);
+      await client.put('/server-settings/chat-providers/access-policy', newPolicy);
     },
     [client, queryClient],
   );

src/workspaces/state/src/useSettings.ts

@@ -49,7 +49,7 @@ export type UseSettingsType = ReturnType<typeof useSettings>;
 
 export type UserSettings = {
   chat: {
-    defaultProvider: 'pi';
+    defaultProvider: 'chat';
     defaultModel: string | null;
     defaultProjectModel: string | null;
     systemPrompt: string;
@@ -63,7 +63,7 @@ export type UserSettings = {
     disabledProviders: string[];
   };
   tasks: {
-    defaultProvider: 'pi';
+    defaultProvider: 'chat';
     defaultModel: string | null;
   };
   appearance: {
@@ -87,7 +87,7 @@ export type UserState = Record<string, unknown>;
 
 export const DEFAULT_SETTINGS: UserSettings = {
   chat: {
-    defaultProvider: 'pi',
+    defaultProvider: 'chat',
     defaultModel: 'claude-code',
     defaultProjectModel: 'claude-code',
     systemPrompt: '',
@@ -100,7 +100,7 @@ export const DEFAULT_SETTINGS: UserSettings = {
     disabledProviders: [],
   },
   tasks: {
-    defaultProvider: 'pi',
+    defaultProvider: 'chat',
     defaultModel: 'claude-code',
   },
   appearance: {