feat(ai-chat): add get_current_page tool for proxy-robust page context (#43, hardness #2)

The current page id was only injected as text in the system prompt, which a
proxy (CLIProxyAPI) can rewrite/truncate, so the agent could lose track of 'this
page'. Add a getCurrentPage tool the model can call to read the open page (id +
title) from the server-side request context (forUser now takes openedPage,
threaded from body.openPage — the same value used for the system prompt). The
inline system-prompt line is kept as belt-and-suspenders. Reads/writes still go
through the CASL-enforced page tools by id, so this is strictly not worse than
the existing prompt hint — just delivered over a channel the proxy can't mangle.

User-approved on the issue. Completes #43 together with the hardness-1 fix.

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

apps/server/src/core/ai-chat/ai-chat.service.ts

@@ -257,6 +257,9 @@ export class AiChatService {
       sessionId,
       workspace.id,
       chatId,
+      // Same open-page value used by the system prompt above; exposed to the
+      // model via getCurrentPage so page identity survives prompt mangling.
+      body.openPage,
     );
 
     // Merge in admin-configured external MCP tools (web search, etc.; §6.8).