fix(pi-subagents): make output artifacts durable and bounded

[tasercake]

Summary

• make child-owned result.log the authoritative result and write fallback output atomically during child session shutdown
• preserve explicit child-written results and provider-error results
• remove all parent result-recovery reads from stdout.log
• stream child session JSONL line by line for crash recovery with bounded per-line memory
• cap stdout diagnostics at 16 MiB and stderr diagnostics at 4 MiB while continuing to drain child pipes
• compact oversized legacy stdout after a result is safely preserved or recovered
• avoid importing the child-only prompt runtime in the parent extension, preventing duplicate lifeline watchers

Validation

• 21 focused tests pass for prompt-runtime persistence, atomic preservation, provider failures, capped logs, oversized stdout, legacy compaction, and streamed large-session recovery
• full Bun compatibility run passes 60 tests; Bun's node:test skip and child-process fd incompatibilities prevent four harness cases from running reliably under Bun
• TypeScript check reaches only the pre-existing agent_settled typing mismatch from the package's older devDependency declarations
• repository CI runs the canonical suite with official Node 22