Add plugin setup and scoped community help

[kreneskyp]

Summary

• Add a plugin-specific setup.md with CLI and local setup checks.
• Point every shipped skill to its setup guide when initialization is missing or an Agent IX command fails.
• Keep Discord community help as a last resort for a reproducible Agent IX product bug; local environment and unrelated project issues stay with their owners.
• Direct Claude Code and Codex plugin installs through the shared agent-ix marketplace.

Publication

• Bump the agent plugin version for host cache refresh.
• The shared marketplace catalog update depends on this source commit and review.
• Restore root plugin manifests needed for the shared marketplace; the skills remain in skills/.