fix(pi-subagents): use dedicated fd for lifeline

[tasercake]

Summary

• move the subagent lifeline from stdin to dedicated fd 3
• preserve stdin semantics for the child Pi process
• use the concrete Node Writable type so the extension type-checks

Validation

• RED: updated lifecycle test failed against stdin/fd 0 implementation
• npm test in extensions/pi-subagents (40 passed, 1 skipped)
• npx tsc --noEmit -p tsconfig.json
• git diff --check