[codex] Summarize MCP denials

[Atomics-hub]

Summary

• add compact redacted denial summaries to blocked MCP tool, resource, and prompt responses
• expose rule, reason, syscall, target, and missing capability without reflecting raw MCP payloads
• extend release-audit smoke checks and docs for reviewer-visible denial evidence

Verification

• cargo fmt --check
• cargo test subprocess_mcp_proxy_mediates_real_stdio_child -- --nocapture
• cargo test subprocess_mcp_proxy_blocks_resource_read_without_capability -- --nocapture
• cargo test subprocess_mcp_proxy_mediates_prompts_list_and_get -- --nocapture
• cargo test release_audit_subprocess_mcp_proxy_smoke_blocks_downstream_execution -- --nocapture
• cargo test in_memory_mcp_proxy_does_not_execute_denied_call -- --nocapture
• cargo test release_audit_subprocess_mcp_proxy_resource_smoke_covers_resource_boundary -- --nocapture
• cargo test release_audit_subprocess_mcp_proxy_prompt_smoke_covers_prompt_boundary -- --nocapture
• cargo test
• cargo clippy --all-targets --all-features -- -D warnings
• cargo run --locked -- release-audit

Note: opened as draft to keep the outage-era local-verified queue explicit; GitHub checks may now be partially restored, but PR #92 remains the first blocked merge candidate.