Add CONTRIBUTING.md and issue/PR templates

[Dannyswiss1]

Add CONTRIBUTING.md (setup instructions, how to run client/, test/lint commands, branch/PR conventions) and
.github/ISSUE_TEMPLATE/ + .github/PULL_REQUEST_TEMPLATE.md to make it easy for new open-source contributors to
pick up issues from this list.

Acceptance criteria

• CONTRIBUTING.md covers local setup, testing, and PR expectations
• Bug report and feature request issue templates added
• PR template includes a test-plan checklist