Chat-First Experience & Redesigned Navigation

**Value proposition**
Agent Builder is evolving from a tool-building interface into a conversation-first platform. Users should be able to open Agent Builder and immediately start chatting and easily expand and extend the capabiliites of the built in Elastic AI Agent without bouncing between multiple pages and components.

**Expected outcome**
A redesigned sidebar-driven navigation that surfaces recent conversations as the primary entry point, with agent configuration (Tools, Skills, Connectors, Plugins) accessible via Customize controls. The agent picker is simplified, and users can seamlessly move between the sidebar chat and a full-screen experience.

**Key user stories**
- As an end user, I want to open Agent Builder and immediately see my recent conversations so I can resume work without hunting through menus.
- As an admin, I want to configure my agent's skills and connections from a single "Customize" panel without leaving the chat context.
- As a user in a Kibana sidebar, I want to expand my conversation to full-screen with a single click.