fix(tui): system prompt not loaded in interactive mode

[avoidwork]

The Problem

In interactive mode (TUI), the system prompt was not being loaded correctly. The agent would fall back to its default identity instead of using the custom persona defined in prompts/SYSTEM_PROMPT.md.

Root Cause

The TUI's handleChat function was calling sessionState.addExchange({ role: 'user', content: text }) before calling dispatchProvider. This meant that when callProvider checked sessionState.getConversation().length === 0 to determine isNewThread, it was already false (the user message was already in the conversation).

Since isNewThread was false, the system prompt was never prepended to the conversation messages in callReactAgent.

The Fix

Move the sessionState.addExchange({ role: 'user' }) call to after dispatchProvider returns, matching the non-interactive flow where callProvider is called before the exchange is persisted.

Before (broken):

sessionState.addExchange({ role: 'user', content: text });  // <-- Too early!
const response = await dispatchProvider(text, ...);
sessionState.addExchange({ role: 'assistant', content: response.content });

After (fixed):

const response = await dispatchProvider(text, ...);  // isNewThread is now true
sessionState.addExchange({ role: 'user', content: text });  // <-- After dispatch
sessionState.addExchange({ role: 'assistant', content: response.content });

Testing

• Non-interactive mode: node index.js "who are you?" - works correctly
• Unit tests: 1117/1118 pass (1 pre-existing failure unrelated to this change)