fix(ci): reject path traversal in fork-controlled diff entries

Copilot finding on .github/workflows/ai-validation.yml:64 — defense
in depth:

  Even though git's tree machinery generally rejects `..` segments
  and absolute paths at commit time, treat fork-controlled diff
  input as untrusted. A path like `docs/../../outside.md` would
  let `git show HEAD:<path>` write to `_changed_md/../../outside.md`,
  which resolves to a sibling of `_changed_md/` — escaping the
  artifact directory and writing into the runner workspace.

Mitigation: validate each path before any mkdir/redirect. Reject:
- absolute paths (`/foo`)
- interior `..` segments (`foo/../bar`)
- leading `..` (`../foo`)
- trailing `..` (`foo/..`)
- bare `..`

Combined with the existing symlink/submodule mode check, the
prepare job now refuses to bundle any tree entry whose path or
type could escape the artifact boundary.

Suppressed-by-Copilot finding (line 68, id-token: write):
Same pushback as previous rounds. claude-code-action@v1 uses OIDC
internally (verified via vyos/vyos-documentation commit b18a399c).
Keeping as-is.

🤖 Generated by [robots](https://vyos.io)

1 files changed, 16 insertions, 0 deletions

diff --git a/.github/workflows/ai-validation.yml b/.github/workflows/ai-validation.yml
index 18671c9d..8e254dcc 100644
--- a/.github/workflows/ai-validation.yml
+++ b/.github/workflows/ai-validation.yml
@@ -54,6 +54,22 @@ jobs:
           # target path, never the target's content.
           mkdir _changed_md
           while IFS= read -r -d '' path; do
+            # Path-traversal hardening: even though git's tree machinery
+            # rejects `..` segments and absolute paths in committed entries
+            # at the porcelain level, treat fork-controlled diff input as
+            # untrusted and validate explicitly. A path like
+            # `docs/../../outside.md` would otherwise let `git show`
+            # write outside _changed_md/.
+            if [[ "$path" == /* \
+               || "$path" == *"/../"* \
+               || "$path" == "../"* \
+               || "$path" == *"/.." \
+               || "$path" == ".." ]]; then
+              echo "::warning::Skipping unsafe path with traversal/absolute prefix: $path"
+              continue
+            fi
+            # Skip non-regular tree entries (symlinks, submodules) per the
+            # commit immediately preceding this one.
             mode=$(git ls-tree HEAD -- "$path" | awk '{print $1}')
             case "$mode" in
               100644|100755) ;;