ci: fail Lint gen on newly created files

[zedkipp]

The "Lint gen" step runs make gen and then git diff --exit-code. That
check only inspects tracked files, so when make gen produces a brand
new Markdown doc (e.g. for a new resource or data source added via
terraform-plugin-docs), the generated file stays untracked and CI
passes while the author's local docs/ is the only place the file
exists.

Reproducing on a clean main:

$ echo "## new" > docs/resources/brand-new-resource.md
$ git diff --exit-code; echo $?
0

Switch to git status --porcelain, which reports modifications and
untracked output, and print the offending paths and diff so the failure
is actionable.

_{Generated by Coder Agents on behalf of @zedkipp.}

Originally discovered this problem here: #501 (comment)