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).

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

@@ -50,6 +50,11 @@ export class AiChatToolsService {
     // agent write (REST + collab) records { actor:'agent', aiChatId } off a
     // SIGNED claim — non-spoofable, never a client body field (§6.5/§6.6).
     aiChatId: string,
+    // The page the user currently has open (from the request context), exposed
+    // to the model via getCurrentPage. Optional and last so existing callers
+    // keep compiling. Kept proxy-robust: the model can CALL for the current
+    // page instead of relying on it surviving in the system prompt text.
+    openedPage?: { id?: string; title?: string } | null,
   ): Promise<Record<string, Tool>> {
     const apiUrl =
       process.env.MCP_DOCMOST_API_URL ||
@@ -210,6 +215,23 @@ export class AiChatToolsService {
         },
       }),
 
+      getCurrentPage: tool({
+        description:
+          'Return the page the user is currently viewing — i.e. what "this page", ' +
+          '"the current page", or "here" refers to. Returns the page id and title, ' +
+          'or null if the user is not currently on a page. Call this first whenever ' +
+          'the user refers to the current page without giving an explicit id.',
+        inputSchema: z.object({}),
+        execute: async () => {
+          if (!openedPage?.id) {
+            return { page: null };
+          }
+          return {
+            page: { id: openedPage.id, title: openedPage.title ?? '' },
+          };
+        },
+      }),
+
       getPage: tool({
         description:
           'Fetch a single page as Markdown by its page id. Returns the page ' +