fix(ui): make child session pages read-only

[tt-a1i]

Summary

• remove the message editor from full-screen child session pages
• keep Direct Subagent and Workflow child pages consistently read-only
• preserve scrolling, follow-to-end, back navigation, and abort controls
• keep parent-model steering through the existing subagent_send tool

Root cause

The shared child page supported an optional editor and Direct Subagent supplied its send callback. Accidental text entered while observing a child was therefore forwarded into the active child session.

Validation

• regression test first reproduced the visible send affordance
• focused child-page tests: 11 passed, 0 failed
• bun run check
• bun run test (Node 889 passed; Vitest 30 passed)
• git diff --check

The installed stable runtime remains unchanged, so no manual TUI smoke is claimed from the modified checkout.