docs: add SECURITY.md community health file

[kitsuyui]

Changes

Add SECURITY.md as a community health file for the gitignore-in organization.

Why

gitignore-in/.github serves as the source of default community health files
for all repositories in the organization. Adding SECURITY.md documents
the vulnerability reporting process and the organization's security posture,
including that secret scanning and push protection are enabled on all repositories.

Scope

• SECURITY.md: New file documenting the security policy and reporting process.
• No changes to existing files.

Verification

• SECURITY.md follows the same structure as other community health files in the organization.
• Secret scanning and push protection are confirmed enabled on the repository.

Trade-offs

• The security features section assumes org-wide settings remain in effect. If settings change, the file should be updated to reflect the current state.