Confine gate standard streams

[chasebryan]

Summary

• replace copied-command stdin with read-only /dev/null
• merge copied stdout and stderr into a write-only pipe relayed by trusted /usr/bin/cat
• assert descriptor identities and access modes at runtime, with protected contract mutations and precise residual-risk documentation

Verification

• scripts/ci/check-repository
• 288 hardened Python tests
• debug and release Rust suites: 162 + 74 + 32 + 9 tests each
• two relocated reproducibility builds and 7 doc tests