fix: Add scope awareness to chat prompt so agents focus on target entities

Pass targetId to buildChatPrompt and add <scope> block that clearly
distinguishes primary target files from context files. Context entities
may be modified when necessary (e.g. dependency links) but the agent
is instructed to focus changes on the primary target.

apps/server/trpc/routers/chat-session.ts

@@ -96,7 +96,7 @@ export function chatSessionProcedures(publicProcedure: ProcedureBuilder) {
           input.initiativeId,
         );
 
-        const prompt = buildChatPrompt(input.targetType, chatHistory, input.message);
+        const prompt = buildChatPrompt(input.targetType, input.targetId, chatHistory, input.message);
 
         // Create a task for the chat agent
         const targetName = input.targetType === 'phase'