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.

1 files changed, 14 insertions, 3 deletions

diff --git a/scripts/doc-linter.py b/scripts/doc-linter.py
index c86ff29a..f580fcdc 100644
--- a/scripts/doc-linter.py
+++ b/scripts/doc-linter.py
@@ -32,12 +32,23 @@ SUPPORTED_EXTS = ('.md', '.rst', '.txt')
 # Linter only applies to published documentation sources under docs/. Repo-root
 # files (README.md, AGENTS.md, .github/copilot-instructions.md) are project
 # meta, not docs content, and are out of scope.
-DOCS_ROOT = 'docs' + os.sep
+DOCS_ROOT = 'docs'
 
 
 def is_docs_path(path):
-    norm = os.path.normpath(path)
-    return norm == 'docs' or norm.startswith(DOCS_ROOT)
+    """Return True iff `path` resolves under the repo's `docs/` tree.
+
+    Accepts both repo-relative and absolute paths. Paths are normalized
+    against the linter's working directory (CI invokes it from the repo
+    root; the same is expected for local runs).
+    """
+    abs_path = os.path.abspath(path)
+    abs_docs = os.path.abspath(DOCS_ROOT)
+    try:
+        return os.path.commonpath([abs_path, abs_docs]) == abs_docs
+    except ValueError:
+        # commonpath raises on mixed drives (Windows) or empty input.
+        return False
 
 # MyST / Markdown fenced code block: leading whitespace + 3+ backticks or 3+ colons.
 # Same character and length-or-greater closes.