Fix Linux sandbox filtered network fail-open

[cursor]

Summary

• Fixes a critical Linux sandbox fail-open where ProxyFiltered domain allowlists were not enforceable under bwrap/firejail. The sandbox previously shared or enabled networking while relying on proxy environment variables, so skill code could bypass the allowlist with direct sockets.
• Adds an explicit Linux network policy validation step that rejects domain-filtered egress before launching a skill, including before weak fallback selection. BlockAll and explicit wildcard direct egress (network_outbound: ["*"]) remain supported.
• Updates EN/ZH architecture docs to document Linux fail-closed behavior for domain allowlists.

Task Linkage

• Task ID: TASK-2026-070
• Task folder: tasks/TASK-2026-070-linux-proxy-fail-closed/

Injected Specs

• spec/architecture-boundaries.md (if architecture/layering changed)
• spec/security-nonnegotiables.md (if sandbox/security changed)
• spec/testing-policy.md (required for any code change)
• spec/docs-sync.md (if behavior/docs/env/commands changed)

Validation Evidence

• Commands executed:
  • cargo fmt --check
  • cargo clippy --all-targets -- -D warnings
  • cargo test -p skilllite-sandbox
  • cargo test
  • python3 scripts/validate_tasks.py
• Key results:
  • cargo fmt --check: passed
  • cargo clippy --all-targets -- -D warnings: Finished dev profile
  • cargo test -p skilllite-sandbox: test result: ok. 84 passed; 0 failed; 0 ignored
  • cargo test: passed through workspace tests/doc-tests
  • python3 scripts/validate_tasks.py: Task validation passed (70 task directories checked).

Regression Scope

• Areas likely affected:
  • Linux sandbox network policy enforcement
  • Skills declaring domain-specific network_outbound on Linux
• Explicit non-goals:
  • macOS Seatbelt behavior
  • Network proxy internals
  • Runtime dependency provisioning

Docs Sync (EN/ZH)

• Not needed
• Updated EN + ZH docs
• Files:
  • docs/en/ARCHITECTURE.md
  • docs/zh/ARCHITECTURE.md

Review Checklist

• Acceptance criteria in tasks/.../TASK.md satisfied (or explicitly deferred)
• tasks/.../STATUS.md updated with latest progress
• tasks/.../REVIEW.md includes merge readiness decision
• tasks/board.md status is up to date