Refresh Northguard Security public profile and research entry points

[Daniele-Cangi]

Changes

• Lead with tangible security engineering and a verified directory of four public research projects.
• Describe the measurement/evidence boundaries without overstating capabilities.
• Preserve the existing banner while improving navigation, collaboration guidance, and authorized-use expectations.
• Add an organization-wide contribution guide with reproducibility and responsible disclosure guidance.

Validation

• All four project links correspond to existing public repositories.
• Existing profile/header.svg was verified in the repository and is unchanged.
• No software behavior, private repository information, credentials, or release metadata are modified.

Docs-only: review GitHub rendering before merge.