Add CI workflow to run tests and vet on every PR and push to dev

[joshuacrass]

Problem:
The only CI workflow in .github/workflows/ is the release pipeline (release.yml). There is no CI check that runs go test ./..., go vet ./..., or a linter on PRs or pushes to the dev branch. Regressions are caught by manual testing only.

Solution:
Add a .github/workflows/ci.yml that runs on push to dev and pull_request targeting dev. At minimum it should run:

• go fmt ./... (fail if output is non-empty)
• go vet ./...
• go test -race ./...

Acceptance Criteria:

• .github/workflows/ci.yml exists and triggers on push to dev and all PRs
• Workflow runs go vet ./...
• Workflow runs go test -race ./...
• Workflow fails the PR if any step fails
• Workflow runs on both ubuntu-latest and macos-latest (cross-platform validation)