fix(tui): preserve session when switching personality

Previously, /personality in the TUI called _reset_session_agent() which
destroyed the agent, cleared conversation history, and effectively started
a new session. This made personality switching disruptive — users lost
their entire conversation context.

Now /personality updates the agent's ephemeral_system_prompt in-place and
injects a pivot marker into the conversation history. The marker tells
the model to adopt the new persona from that point forward, which is
necessary because LLMs tend to pattern-match their prior responses and
continue the established tone without an explicit signal.

Changes:
- tui_gateway/server.py: Rewrite _apply_personality_to_session to update
  the agent in-place instead of resetting. Inject a user-role pivot
  marker so the model actually switches style mid-conversation.
- ui-tui/src/app/slash/commands/session.ts: Update help text (no longer
  mentions history reset).
- tests/test_tui_gateway_server.py: Update test to verify history is
  preserved, pivot marker is injected, and ephemeral prompt is set.

ui-tui/src/app/slash/commands/session.ts

@@ -109,7 +109,7 @@ export const sessionCommands: SlashCommand[] = [
   },
 
   {
-    help: 'switch or reset personality (history reset on set)',
+    help: 'switch personality for this session',
     name: 'personality',
     run: (arg, ctx) => {
       if (!arg) {