fix(tui): isolate background subagent session events

[kingsword09]

Background subagents can stream tool cards and final-answer text into the foreground because the runtime broadcasts raw child-session events alongside parent-session tool mirrors.

Preserve session ownership during event normalization and reject events from other sessions at both TUI event entry points. Treat inputSource: "subagent" turns as task-scoped without starting a parent result handoff, so foreground input handling stays independent of child execution. Parent TaskOutput results and background completion notifications remain visible.

Regression coverage injects two child sessions' text, Bash activity, protocol parts, and nested-task notifications, then checks both the normal and expanded foreground transcript while preserving a parent TaskOutput result.

Validation:

• bun test --dots: 612 passed, 0 failed
• bun run typecheck
• bun run build
• Locked runtime sync and bun scripts/check-runtime.ts
• bun run check:tui
• bun scripts/check-package.ts
• git diff --check

Fixes #129