Add SageOx in Practice, streamline contributing & README

[port8080]

Summary

• README: Add "SageOx in Practice" subsection showing PR replace flock-based daemon liveness with socket-ping detection #4 as a real-world example of the team context → implementation → review loop, with a Mermaid flowchart. Remove "How Ox Fits In" and "Supported AI Agents" sections to keep it focused.
• CONTRIBUTING.md: Adopt issue-driven contribution model with community attribution rule
• demo/README.md: Trim to internal re-recording instructions only
• CLAUDE.md: Add community attribution rule for issue-driven contributions

Test plan

• Verify Mermaid diagram renders on GitHub
• Confirm PR replace flock-based daemon liveness with socket-ping detection #4 link works: replace flock-based daemon liveness with socket-ping detection #4
• Review README flow reads naturally after removed sections