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

Follow-up on the now-merged #1957 (ubuntu-latest switch). CodeRabbit
raised these findings on the paired add-to-circinus PR #1959 (where
the same file was being added to the circinus branch); since the file
contents are byte-identical across rolling/circinus/sagitta, applying
the same fixes here.

  1. SHA-pin actions/checkout@v6 (x4), actions/upload-artifact@v4,
     actions/download-artifact@v4, anthropics/claude-code-action@v1.
     pull_request_target has secrets + repo write — GitHub security
     guidance recommends full commit SHAs as the only immutable
     release form.

  2. Reject paths containing control characters (NUL/CR/LF) in
     changed-md.z and changed-rst.z before `tr '\0' '\n'` converts
     them to newline-delimited manifests. A fork PR committing
     `docs/foo<LF>bar.md` would otherwise split into two logical
     lines, masking the real file from line-based consumers.

  3. Pin reference-DB download to `tag: ${{ env.REVIEWER_REF }}`
     (was `latest: true`). Aligns DB version with the pinned
     reviewer code; a future reviewer-v1.x.x release with a DB schema
     change can't be silently picked up.

  4. Drop `id-token: write` from validate job permissions. No OIDC
     usage; copy-paste leftover.

Paired PRs on release branches (byte-identical file contents):
  * circinus: #1959 (commit 025319ea)
  * sagitta:  #1960 (commit e5506317)

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