Add org profile, community health defaults and reusable .NET workflows

[vfofanov]

Organization defaults for every DragoAnt repository.

• Profile — profile/README.md, the org front page with the package table.
• Community health defaults — CONTRIBUTING.md, SECURITY.md (private vulnerability reporting), CODE_OF_CONDUCT.md (Contributor Covenant 2.1), SUPPORT.md, issue forms (bug, feature, config) and a PR template. No FUNDING.yml: neither the org nor the owner has a Sponsors listing.
• Reusable workflow .github/workflows/dotnet-build.yml — restore → build → test on Microsoft.Testing.Platform (TRX + JUnit + Cobertura) → test table and coverage summary in the job summary, optional coverage threshold → test-results, coverage, packages artifacts. Solution-file agnostic (*.slnx, then *.sln).
• Workflow templates — ci and release starters (with .properties.json) for Actions → New workflow. They reference the reusable workflow @main; pin a SHA after adding.
• CI — actionlint on push/PR; Dependabot for actions.

Every action is pinned to a full commit SHA (all node24); jobs run with contents: read only.

Follow-ups after merge

• DragoAnt/Extensions.System.Text.Json ci.yml/release.yml reference this branch's head SHA — re-point them to the merged main SHA.
• CODE_OF_CONDUCT.md routes reports through the private vulnerability form or GitHub's abuse report; swap in a dedicated e-mail if you get one.
• Optional: enable Require actions to be pinned to a full-length commit SHA in the org Actions settings.