ci(ai-validation): address 4 CR findings — SHA pins, NUL guard, DB pin, drop id-token

Addresses the four CodeRabbit `Major / Quick win` findings raised
on this PR (after flip to ready):

1. SHA-pin all action invocations (was movable tags `@v6`, `@v4`, `@v1`
   on 4 actions). pull_request_target workflows have repo write access
   and access to secrets; GitHub security guidance recommends full
   commit SHAs as the only immutable release form. Other steps already
   were SHA-pinned, so this just resolves the inconsistency:
     * actions/checkout@v6                 -> @de0fac2e # v6.0.2 (x4)
     * actions/upload-artifact@v4          -> @ea165f8d # v4.6.2
     * actions/download-artifact@v4        -> @d3f86a10 # v4.3.0
     * anthropics/claude-code-action@v1    -> @476e359e # v1.0.119

2. Reject paths containing control characters (NUL/CR/newline/etc) in
   changed-md.z and changed-rst.z before `tr '\0' '\n'` converts them
   to newline-delimited manifests. A fork PR could in principle commit
   `docs/foo<LF>bar.md` and split it into two logical lines, masking
   the real file from line-based consumers. The filesystem and
   porcelain git typically reject these but the path could survive in
   the tree object; fail-fast in the workflow.

3. Pin reference-DB download to `tag: ${{ env.REVIEWER_REF }}` (was
   `latest: true`). REVIEWER_REF already fixes the reviewer code to
   `reviewer-v1.0.1` on every use; the DB download was the one place
   that resolved from the latest release tag, breaking reproducibility
   and creating a silent compat-window where a future v1.x.x release
   with a DB-schema change could be picked up while pinned reviewer
   code still expects the old schema.

4. Drop `id-token: write` from the validate job's permissions. No step
   in the workflow uses OIDC; the permission was a copy-paste leftover.
   Least-privilege.

Files: same change copied byte-identically to all three workflow PRs
once they land (this PR for circinus, paired PR for sagitta, follow-up
on rolling since #1957 already merged).

0 files changed, 0 insertions, 0 deletions