chore: add devcontainer contributor workflow [DX-822]

[Tyler Collins (t-col)]

This repository still relied on a legacy CI setup and did not have a checked-in devcontainer, which meant contributors had to reconstruct the right toolchain locally and had no guarantee that their machine matched the environment used in automation. The practical effect is that routine fixes and feature work are harder to start, and environment drift becomes a source of avoidable failure.

This change makes the repository-local devcontainer the source of truth for both local development and CI. It adds .devcontainer/Dockerfile and .devcontainer/devcontainer.json, adds a GitHub Actions workflow built around devcontainers/ci, removes the retired CircleCI or Travis build configuration, and updates the public contributor documentation and badges so the documented path matches the checked-in tooling. CodeQL is left in place.

This keeps the existing Ruby contributor workflow intact while making the containerized setup the source of truth for both local work and CI.

Validation for this repo was done against the checked-in devcontainer configuration rather than a hand-built local environment. Built the devcontainer and ran bundle install && bundle exec rake rspec_rubocop. I also rechecked the updated devcontainer JSON and workflow YAML for validity and confirmed that stale CircleCI/Travis references were removed from the public README and CONTRIBUTING entry points.

[wiz-inc-38d59fb8d7]

Wiz Scan Summary

Scanner	Findings
Vulnerabilities	-
Sensitive Data	-
Secrets	-
IaC Misconfigurations	1
SAST Findings	-
Software Management Findings	-
Total	1

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