fix(pi-subagents): let successful children exit with lifeline active

[tasercake]

Summary

• wrap the child lifeline fd in an unref'd net.Socket instead of a blocking fs.ReadStream
• preserve EOF-based parent-death and recursive termination behavior
• add regressions for natural child exit and active-child termination on parent EOF

Tests

• node --experimental-strip-types --test test/unit/*.test.ts (Node v24.18.0): 55 passed, 1 skipped
• tsc --noEmit -p tsconfig.json: existing baseline errors only; no new diagnostics