repo: add CI badge, CODEOWNERS, and strict PR template

[gkalombo21]

Pull Request Template

Description

Short description of the change.

Linked Issue

Closes #

Scope Confirmation

• Targets dev branch
• Linked to an open issue
• Does not modify consensus logic (unless explicitly scoped)
• Does not introduce new cryptographic primitives (unless explicitly scoped)

Security Checklist

• I understand all code submitted
• I reviewed overflow/underflow risks
• I reviewed error handling paths
• I added unit tests
• I added edge-case tests where applicable

Security-sensitive changes (crypto, consensus, state transitions, economic logic) may require extended review.

AI Usage Disclosure

• I used AI assistance
• I did NOT use AI assistance

If AI was used, briefly describe how (boilerplate, refactoring, tests, etc.):

Responsibility

By submitting this PR, you confirm you understand and take responsibility for the code.