doc-linter: realpath() resolution, DOCS_ROOT in walker, indent fix

Three Copilot findings on ab497bf:

1. is_docs_path() docstring claimed paths 'resolve' under docs/, but
   the implementation only normalized via abspath() — a symlink under
   docs/ that points outside the tree would be treated as in-scope.
   Switch both inputs to os.path.realpath() so symlinks are followed
   to their real targets. The reverse case is also handled: if docs/
   is itself a symlink (some CI checkouts), realpath() resolves it
   consistently for both sides of the commonpath comparison.

   Verified with a synthetic case: docs/poison.md -> /etc/hosts now
   returns False (with abspath() it returned True).

2. The auto-discover fallback in main() still hardcoded
   os.walk('docs') instead of using the new DOCS_ROOT constant.
   Use DOCS_ROOT in both paths so the docs root is configured in
   exactly one place.

3. Indentation inside 'for file in files:' was double-indented (8
   spaces under the for, instead of 4) — pre-existing oddity from
   before 65a8e9f, preserved through the is_docs_path() addition.
   Normalize to a single indent level under the loop.

CI behavior unchanged: tj-actions/changed-files passes repo-relative
paths with no symlinks under docs/, which were already handled. The
realpath() switch only changes behavior in the symlink-escape case,
which was a bug.

0 files changed, 0 insertions, 0 deletions