Out-of-process delegation to external coding agents as two new subagent
seam backends, exposed as subagent_claude_code / subagent_codex tools.
Verified against @anthropic-ai/claude-agent-sdk 0.3.202 and codex CLI
0.142.5 via keyless spikes; includes the dsh-subagent-process extraction
plan, isolation/permission stances, and tiered test coverage.