feat(render): add autogenerated notice to workflow files

[lovesegfault]

Summary

• Generated YAML workflow files now include a header comment explaining they are autogenerated
• The header includes instructions on where to find the source configuration in the flake
• Users are shown how to regenerate via nix run .#render-workflows or the pre-commit hook

Example output

# This file was autogenerated by actions.nix. Do not edit it manually.
# To make changes, edit the workflow definition in your flake's actions-nix configuration
# (typically under flake.actions-nix.workflows.".github/workflows/main.yaml") and run:
#   nix run .#render-workflows
# Or commit to trigger the pre-commit hook if enabled.
jobs:
  ...

Test plan

• Ran nix run .#render-workflows and verified the header appears in generated files
• Pre-commit hooks pass

[nialov]

Thanks, merged!