Add public-safe visual assets plan

[keepittechie]

Summary

• Added a public-safe visual assets guide
• Added an assets index with placeholder folders
• Updated screenshot policy and pre-publish review guidance
• Linked visual asset guidance from README and docs navigation
• Added ignore rules for raw image-editing project files

Safety Review

• Markdown link check passed
• Diagram structure check passed
• Public-safety scan passed
• git diff --check passed
• Private domain scan returned clean
• Personal path/email scan returned clean
• Private-language scan returned clean
• No real screenshots, dashboard captures, exact host IPs, private domains, raw exports, credentials, or private data were added

Notes

This PR only adds guidance and placeholder asset folders. It does not add actual screenshots or visual captures.