fix(sandbox): avoid nested deny mount failures

[ZhiXiao-Lin]

Summary

• collapse redundant SRT deny-write descendants already covered by a protected ancestor
• preserve credential read denial and standalone sensitive-file write protection
• align Core, Node, Python, and bootstrap release metadata at 6.3.1
• keep the escalated Bash justification guard compatible with current Clippy without changing behavior

Why

The CLI release sandbox probe failed on Linux when .a3s was read-only and SRT then tried to create a nested .a3s/os-auth.json mount point. Removing only the redundant child write deny lets the sandbox start while the parent write boundary and child read boundary remain fail-closed.

Validation

• cargo fmt --all -- --check
• cargo clippy -p a3s-code-core --lib -- -D warnings
• cargo test -p a3s-code-core --lib (2528 passed, 14 ignored)
• cargo check --manifest-path sdk/node/Cargo.toml
• cargo check --manifest-path sdk/python/Cargo.toml
• bash check-version.sh 6.3.1
• event protocol artifact check
• SDK API alignment check