ci: evaluate making GitHub lint checks required

## Problem

PR #252 restores the GitHub `Lint` workflow and proves the PowerShell, Markdown, and YAML jobs complete successfully. Those checks are currently advisory: the `main` ruleset requires `connectors-sdk.public`, but not the GitHub lint jobs. A red lint result can therefore be merged after approval if the required build remains green.

## Proposed follow-up

After #252 merges and the check names are stable:

- Evaluate adding the PowerShell, Markdown, and YAML GitHub lint checks to the `main` ruleset.
- Confirm required-check names and strict update behavior using a test PR before enforcing them.
- Document whether Azure Pipelines remains the authoritative C# and package-validation gate.

This is a repository-governance change and should be coordinated with `connectors-sdk-maintainers`.

Follow-up from #252.