fix(ci): address Copilot review on PR #1947

Three Copilot findings on the AI Validation workflow:

1. line 46 (cp loop on deleted files):
   `git diff --name-only` with no filter includes Deleted entries, so
   the subsequent `xargs -0 ... cp --parents` would fail when a PR
   deletes (or renames) a `.md`/`.rst` file. Added
   `--diff-filter=ACMRT` to both name-only diffs so deletions are
   excluded from the cp source list. Deletions still appear in
   diff-md.patch (which uses the unfiltered full diff) so Pass 1's
   --pr-diff input still sees them via the patch hunk.

2. line 103 (reviewer sparse-checkout):
   The App token was being persisted into reviewer/.git/config as
   an http extraheader by default. The Pass 2 claude-code-action
   step has Read/Glob/Grep allowed, so a prompt-injection attempt
   could exfiltrate the token from the workspace. Added
   `persist-credentials: false`. The sparse-checkout fetched only
   branches.json which is a one-shot read, no further git ops
   needed in this job.

3. line 127 (vyos-1x checkout):
   Same persist-credentials concern. The vyos-1x clone is read by
   claude-code-action for Pass 2 source inspection — exactly the
   step where filesystem read tools are exposed to LLM-driven
   shell behavior. Added `persist-credentials: false`. The vyos-1x
   tree is only read after this checkout (`Read,Glob,Grep` over
   `.vyos-1x/`); we don't run any git commands against it that
   would need the token.

Suppressed-by-Copilot finding (line 64, id-token: write):
Pushback. claude-code-action@v1 uses OIDC internally (verified by
commit b18a399c on the previously-deployed workflow). Removing
this permission breaks the action. Keeping as-is.

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

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