Add experimental model router packaging

[nvk]

Summary

• Add a dormant shared model-router reference and placeholder example config.
• Add stdlib-only scripts/wiki-router helper for status, chat, embed, rerank dry-runs, and optional live OpenAI-compatible calls.
• Package the reference through Codex/OpenCode mirrors and add dependency-free validation tests.

Tests

• ./tests/test-model-router-disabled.sh
• ./tests/test-model-router-config-parse.sh
• ./tests/test-plugin-validate.sh
• python3 -m py_compile scripts/wiki-router
• git diff --check
• ./tests/test-structure.sh
• ./tests/test-codex-sync.sh
• ./tests/test-opencode-sync.sh