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/servers/api/server-settings/server-settings.ts

@@ -3,7 +3,7 @@ import { readdirSync, existsSync } from 'node:fs';
 import { join } from 'node:path';
 import { readServerSettings, writeServerSettings } from 'officerdb';
 import { claudeCodeRouter } from './claude-code';
-import { piMonoRouter } from './pi-mono';
+import { chatProvidersRouter } from './chat-providers';
 import { applicationsRouter } from './applications';
 import { smtpRouter } from './smtp';
 import { ttsRouter } from './tts';
@@ -13,7 +13,7 @@ import { ocrRouter } from './ocr';
 export const serverSettingsRouter = createRouter();
 
 serverSettingsRouter.route('/claude-code', claudeCodeRouter);
-serverSettingsRouter.route('/pi-mono', piMonoRouter);
+serverSettingsRouter.route('/chat-providers', chatProvidersRouter);
 serverSettingsRouter.route('/applications', applicationsRouter);
 serverSettingsRouter.route('/smtp', smtpRouter);
 serverSettingsRouter.route('/tts', ttsRouter);