docs: welcome open-source contributors

[unipheas]

What changed

• make the MIT rights to use, fork, modify, redistribute, and sell explicit
• add contribution and private security-reporting guides
• add structured bug, feature-request, and pull-request templates
• ignore local secrets, signing artifacts, and security reports by default

Why

BarKeep is a public open-source project, so its licensing, contribution path,
and security-reporting process should be obvious before anyone forks it.

Verification

• canonical MIT license text verified
• relative Markdown links checked
• issue-template YAML parsed
• credential-pattern scan passed
• Swift test suite passed: 24 tests
• release build completed