XY-858: Wire natural-language research trigger into Decodex plugin surface

[yvette-carlisle]

Summary

• Route ordinary natural-language research X requests into Decodex-native research/design intake with latent Decision Contracts.
• Document promotion follow-ups such as arrange this, push this forward, 推进, and 做 as the boundary before planning and execution authority.
• Preserve manual CLI, automation, labels, commit, and land skill boundaries while adding packaged plugin content tests.

Validation

• cargo make fmt
• cargo make lint-fix
• cargo make test
• cargo test -p decodex plugin_surface_tests