Remove GitHub Actions

[becker63]

What changed

• removed all three GitHub Actions workflow files
• removed the workflow generator, drift checker, Actions secret installer, and actionlint integration
• updated Nx, Nix, dependency, and Repo Insight documentation/configuration so local checks no longer expect those workflows

Why

The repository should no longer execute GitHub Actions. Removing both the checked-in workflows and their generators prevents them from being triggered or recreated accidentally.

Impact

After this PR is merged into main, GitHub will no longer have workflow definitions to trigger from the default branch. Existing Actions run history will remain visible, and any already queued or running job may need to be cancelled separately.

Local Repo Insight commands and CI-style Nx checks remain available.

Validation

• bun install --frozen-lockfile --ignore-scripts
• bunx nx run workspace:ci-automation
• bunx nx run workspace:lint
• nix flake check --no-build
• pre-commit typecheck and lint hooks