fix(ci): bundle .md via git-show blobs, not filesystem cp (symlink exfil)

Copilot finding on .github/workflows/ai-validation.yml:50 — high severity:

  A fork PR can commit a symlink at docs/x.md (mode 120000) pointing to an
  absolute path on the self-hosted runner. The previous `xargs cp` would
  follow the symlink and copy the *target's* content (/etc/passwd, runner
  secrets, ssh keys, the App's PEM if accessible) into the pr-input
  artifact. The validate job downloads that artifact and exposes it to the
  Pass 2 claude-code-action step (which has Read,Glob,Grep tools), so a
  prompt-injection attempt could exfiltrate runner state via inline review
  comments.

Mitigation: replace the cp loop with `git show HEAD:<path>` extraction.
This pulls bytes directly from the merge commit's tree blob. For a symlink
entry, git show returns the textual target path (a string like
"/etc/passwd"), NOT the target's filesystem content. The artifact's
worst-case is a text file containing a path string, which has no
exfiltration value.

Implementation:
- Read NUL-delimited paths from changed-md.z (preserves filename safety).
- For each path, check ls-tree mode: 100644/100755 = normal file, accept;
  120000 = symlink, skip with ::warning::; 160000 = submodule, skip;
  anything else, skip.
- Use `git show HEAD:<path>` to write blob content into _changed_md/.
- All variable expansions are within double-quotes (no word-splitting,
  no glob, no recursive parse of $() in the substituted value).

The xargs cp option-injection defense (`-- ` end-of-options) is no longer
needed since cp is gone. The --diff-filter=ACMRT and persist-credentials
fixes from edd903d remain.

Suppressed-by-Copilot finding (line 68, id-token: write):
Same as previous round — pushback. claude-code-action@v1 uses OIDC
internally (verified via vyos/vyos-documentation commit b18a399c, where
the permission was removed and immediately restored after the action
broke). Keeping as-is.

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

0 files changed, 0 insertions, 0 deletions