Add project governance files for a reusable template

[Subhransu-De]

What to build

Add basic governance files so the template can be reused or published with clear expectations.

Acceptance criteria

• Add a license file appropriate for the intended reuse model.
• Add SECURITY.md with vulnerability reporting guidance.
• Add CONTRIBUTING.md with local setup, checks, and contribution expectations.
• README links to the governance files.

Blocked by

None - can start immediately.