fix(tui): open workflow children as full pages

[tt-a1i]

Summary

• add one shared full-terminal Child Session Page for Direct Subagents and Workflow children
• remove the bordered Workflow transcript panel while preserving its read-only lifecycle
• keep Direct steer input and make both paths share rendering, follow-to-end, pause/resume, and back navigation
• retain bounded transcript evidence without fabricating Pi native expanded tool state

Validation

• bun run check
• bun run test (Node 886 passed; Vitest 30 passed)
• git diff --check
• Standards review: no actionable findings
• Spec review: no code findings after completing the requested real-TUI smoke
• isolated real-TUI smoke: Workflow and Direct both cover the full terminal; default follow, manual-scroll pause, G resume, and Esc return verified

Corrective follow-up to #106.

Closes #105