feat: add six public skill packets

[xiaojiou176]

Summary

• add public skill packets for OpenUI, Movi, Switchyard, ProofTrail, Shopflow, and Prooflane
• register all six skills in marketplaces/default.json
• keep the packets capability-first, self-contained, and honest about local MCP boundaries

Validation

• copied the current repo-owned public skill packets into OpenHands/extensions-ready skill folders
• validated marketplaces/default.json remains valid JSON

Notes

• these packets are install/config/use guides for repo-native MCP surfaces, not claims of hosted services or official marketplace parity

[xiaojiou176]

Superseded by #161, which carries the reviewer-facing capability-first rewrite and the current packet set for these six skills. Closing this earlier draft to keep the review surface on a single PR.