feat: add subagent orchestration kit

[Asheze1127]

Summary

• add a repo-local plugin entry for reusable sub-agent workflows
• add a subagent-orchestration-kit plugin with role definitions for idea consulting, coding, evaluation, review, and drift checking
• add HackHub-specific product context plus a portable project overview template for reuse in other repositories
• clarify that sibling-repo doc paths are specific to this workspace and must be replaced when reusing the plugin elsewhere

Validation

• python3 -m json.tool .agents/plugins/marketplace.json
• python3 -m json.tool plugins/subagent-orchestration-kit/.codex-plugin/plugin.json

Notes

• this plugin packages reusable prompts and operating docs; it does not add new first-class agent types to Codex
• keeping this as a draft PR for manual review before anything lands on dev

[Asheze1127]

Opened as the clean replacement for #51. This branch is based directly on origin/dev and contains only the plugin files.