feat(tui): unify child transcript rendering

[tt-a1i]

Summary

• render Direct Subagent and Workflow child conversations through one shared transcript renderer
• share follow-to-end viewport semantics while preserving each surface’s navigation and input behavior
• reuse the parent tool activity projection for child tool rows and preserve legacy Workflow artifacts

Validation

• bun run check
• bun run test (Node 884 passed; Vitest 30 passed)
• git diff --check

Closes #105