fix(pi-subagents): prevent oversized stdout string failures

[tasercake]

Summary

• restore child Pi print mode instead of persisting cumulative JSON streaming events
• rely on the child prompt runtime for provider-error exit handling
• bound legacy stdout, stderr, and session recovery reads at 16 MiB
• preserve existing results and terminalize dead records without throwing on oversized logs

Regression

Commit 5deb747 enabled JSON mode. Pi JSON message_update events include cumulative message state, producing stdout logs larger than V8's maximum string length when finalization called readFileSync(..., "utf-8").

Validation

• 8 focused Bun tests pass (print args, provider failures, autosave including valid JSON text, preserved results, oversized reconciliation)
• full Bun compatibility run: 56 tests pass; two pre-existing harness/runtime incompatibilities remain (node:test skip under Bun and the known duplicate lifeline watcher case)
• repository CI uses official Node 22 and runs the package's npm test script