[codex] Refactor dynamic worker error transport with Effect causes

[heyglassy]

Summary

• replace the dynamic worker's string-only error transport with a structured internal envelope derived from Effect causes
• preserve the existing outward execution contract and user-facing error strings while keeping structured failures intact across the worker boundary
• rethrow structured tool failures through the worker transport so plain objects render as JSON instead of collapsing to [object Object]

Root Cause

The dynamic worker runtime flattened failures at the Workers RPC boundary with ad hoc string conversion. That discarded Effect cause structure and collapsed object-shaped failures to [object Object] before the execution engine could format them.

Validation

• cd packages/kernel/runtime-dynamic-worker && bun x vitest run

Notes

• supersedes [codex] Improve dynamic worker error formatting #241
• this is a clean refactor from main, not a follow-up on top of the minimal-fix branch

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commit: 4ed92df