feat(pm): supervised-child runtime helper — pipe-EOF liveness + descendant cleanup [PM-B]

[rickylabs]

Part of #510.

Core runtime helper (the PM-16 sd_notify-helper pattern): in supervised mode a child watches its inherited stdio pipe; EOF (supervisor death, incl. hard kill) → bounded self-termination + descendant cleanup. Consumed thinly by plugin ./services entrypoints and by the guardian wrapper for raw executables — the universal containment layer (RFC §4-F1). Depends on PM-5 #516.

• gate: EOF → bounded self-termination unit tests, both platforms
• gate: public pm-core runtime export — @module doc + runnable example, jsr rubric

Design source: PR #822 (rfc.md + plan.md rev 10, 9-cycle adversarial trail) in .llm/runs/rfc-single-deployment--orchestrator/. (plan.md §A.3)