fix(design): block validator egress and harden review key exposure

[echobt]

Summary

• Add validator to Design egress BLOCKED_HOSTNAMES (defense-in-depth alongside post-DNS IP deny).
• Stop putting OPENROUTER_API_KEY in the design-review container environ; mount it as OPENROUTER_API_KEY_FILE under /run/review-secrets (Linux /proc/<pid>/environ is a boot-time snapshot and cannot be scrubbed by unsetenv).
• Harden run_command: keep child env_clear, deny procfs and review-secrets paths. Document that AGENTIC_ENABLE_RUN_COMMAND=1 stays on in prod with these mitigations.

Test plan

• cargo test -p design-egress-proxy -p challenge-agentic -p review-docker --lib
• cargo clippy -p design-egress-proxy -p challenge-agentic -p review-docker -p challenge-review-bin --all-targets -- -D warnings
• cargo run -p xtask -- design-check
• After deploy: confirm review containers receive OPENROUTER_API_KEY_FILE (not OPENROUTER_API_KEY=) and egress still blocks validator by name

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 3b83cfdf-28cc-478c-9080-377452342ff2

📥 Commits

Reviewing files that changed from the base of the PR and between 966102d and 1e9505a.

📒 Files selected for processing (6)

• bins/challenge-review/src/main.rs
• crates/challenge-agentic/src/tools.rs
• crates/design-egress-proxy/src/lib.rs
• crates/review-docker/src/lib.rs
• docs/DESIGN_CHALLENGE.md
• docs/OPERATOR_SECURITY.md

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