CLI: Automatic V2 Agent Upgrades and ACP Sign-In

This release turns on automatic V2 agent upgrades by default when a V3 session starts and lets ACP clients that declare terminal authentication sign in to Kiro from a terminal. Custom agents built for V2 keep working on V3 without a manual upgrade run, and a capable client opens a terminal sign-in flow. It also draws Mermaid node shapes and nested subgraphs, adds a KIRORUNTIMEDIR override for the bundled runtime, and tidies V3 approval prompts. Fixes cover fullscreen scrolling, click targeting in fullscreen, and the Homebrew upgrade hint.