feat(candidate): freeze model gateway access

[drewstone]

Summary

• bind model-calling candidate plans to exact evaluator-approved gateway domains
• keep general candidate network access disabled
• reject wildcards, URLs, private hosts, unsorted domains, and gateway access on zero-call plans

This closes the live-execution gap found by the Pier zero-call controls: model-backed candidates can receive a narrowly filtered endpoint without gaining arbitrary internet access.

Verification

• pnpm --filter @tangle-network/agent-interface check-types
• pnpm --filter @tangle-network/agent-interface test (103/103)
• pnpm --filter @tangle-network/agent-interface build
• git diff --check
• clean merge-tree against origin/main