feat(tui): group sequential thinking - #36901

Summary

• group adjacent reasoning spans while preserving text and tool boundaries
• keep the latest complete title stable during streaming and summarize completed runs with title, step count, and duration
• render expanded reasoning as flat Markdown blocks with title · content and one blank line between items
• migrate affected dialog test harnesses to the V2 keymap provider and restore the prompt submit default inherited from the current v2 base

Demo

▶ Watch the OpenTUI interaction

Checks

• GOMAXPROCS=2 bun typecheck (packages/tui)
• bun test --only-failures (packages/tui: 224 pass, 1 skip)
• bun test test/cli/tui/session-rows.test.ts (10 pass)