fix(subagent): harden structured result channel

[Grivn]

Summary

• replace the DSH child outputSchema dependency with a per-run UUID-named inherited result tool whose parameters match the exact Mnemon operation schema
• preserve strict tool allowlists while validating child ownership, terminal completion, nested code-mode results, duplicate calls, and fail-closed cleanup
• expand host execution contracts, regression coverage, and bilingual documentation without changing DSH

Validation

• pnpm run verify: 334 passed, 1 skipped; deterministic build, headless activation, package verification, publint, and attw passed
• WebUI Memory → Search → Agent Query returned a grounded answer with citations and raw evidence
• WebUI conversation mnemon_recall spawned a Mnemon recall child, called the allowed read tools, invoked mnemon_subagent_result_, returned Mnemon subagent result recorded, and delivered the structured result to the parent Agent
• git diff --check passed

This works around the DSH rc.6 structured_output/tool-filter interaction described in #14 while keeping the compatibility fix entirely inside dsh-mnemon.