Add repository hygiene baseline and contributor DX tooling

[lipkau]

Problem

Repository scaffolding has drifted behind current AtlassianPS standards, increasing onboarding friction and configuration inconsistency.

Proposal

Add and align repository hygiene files and tooling with current practice:

• .editorconfig
• .devcontainer setup
• Dependabot configuration for workflows/devcontainer dependencies
• README badge and contributor docs updates to current CI/CD approach

Acceptance criteria

• .editorconfig is added and applied to key file types
• .devcontainer exists and can bootstrap contributor environment
• .github/dependabot.yml covers relevant ecosystems
• README build/status badges point to active GitHub workflows
• Contributor guidance reflects current development/testing flow