Keep native integration details out of TypeRB

[hiroki23]

Summary

• follow TypeRB #590 by invoking the consumer-neutral compiler bootstrap-snapshot command and matching its diagnostics
• pin the reference compiler and CI checkout to merged TypeRB revision 920f34547723c58afe62f5f180fe0892f0a333c1
• make the cross-repository ownership invariant explicit in AGENTS, Architecture, Contributing, and the Native development skill
• keep Native gate mappings, integration commands, revision pins, and bridge lifecycle policy in this repository

Validation

• trb fmt --check .
• trb check
• trb check for both benchmark tools and all Gate 1/2 corpus configurations
• full TypeRB Native test suite with the pinned reference compiler and QBE 1.3: 39 tests, 0 failures
• quick_validate.py .agents/skills/develop-typerb-native
• git diff --check