Document shared SAGE governance

[ShuhaoZhangTony]

Summary

• define SAGE as the IntelliStream ecosystem shared flagship product and technical vision
• document RIDE Lab principal core stewardship without implying exclusive ownership
• update canonical repository, documentation, installer, and contribution links after the organization rename

Validation

• YAML, JSON, and TOML contract checks
• changed shell scripts pass syntax validation
• satellite repository manifest test passes
• installation consistency check passes with environment warnings
• diff and retired-owner reference scans pass