doc-linter: handle absolute paths in is_docs_path()

CodeRabbit review on 28224f3 flagged that is_docs_path() introduced
in 65a8e9f only matched repo-relative path strings. An absolute path
to docs/... (e.g., from a local invocation that pre-resolves paths,
or from tooling that uses git ls-files --full-path) would silently
fail the docs/ check and the file would be skipped.

Rewrite the helper to use os.path.commonpath against an absolute
docs/ root computed on each call. Both inputs are normalized to
absolute form, so repo-relative and absolute callers produce the
same result. ValueError from commonpath (mixed Windows drives or
empty input) is caught and treated as 'not a docs path'.

abs_docs is recomputed per call rather than captured at import time
so the helper picks up the actual cwd at invocation, matching the
existing assumption that CI / local runs invoke the linter from the
repo root.

Verified against 12 edge cases:
- repo-relative docs paths (docs, docs/foo.md, docs/sub/dir/foo.md,
  ./docs/foo.md) -> True.
- repo-relative meta paths (AGENTS.md, README.md,
  .github/copilot-instructions.md, docs_other/foo.md) -> False.
- absolute paths inside docs/ -> True; inside repo root but outside
  docs/ -> False.
- traversal attempts (../other/foo.md, docs/../AGENTS.md) -> False.

CI behavior unchanged: tj-actions/changed-files passes repo-relative
paths, which were already handled by the previous logic.

0 files changed, 0 insertions, 0 deletions