ci(ai-validation): 3 CR/Copilot follow-ups — NUL guard restore + issues: write + fail-closed gate

CodeRabbit / Copilot findings on the merged PR #1959 and the still-open
PRs #1960 + #1969:

1. NUL/control-char guard regressed. PR #1959 (merged into circinus
   without this) — and likewise sagitta/rolling-followup — had the
   guard inadvertently dropped during the rolling@HEAD rebase, because
   rolling@HEAD never had the corrected form. Restoring the
   user-corrected form: `LC_ALL=C grep -zPq "[\\x01-\\x1F\\x7F]"`
   reads the NUL-delimited *.z files directly (NUL is the record
   delimiter, not a forbidden byte) and rejects every other control
   byte 0x01-0x1F + 0x7F. An earlier `tr -d "\\0\\n\\r" | grep ...`
   form stripped the very bytes it was meant to reject before the
   grep ran — see the inline comment block for the contract.

2. `gh pr comment` requires `issues: write`. The skip-notice step and
   Pass 2 summary comment both post via `POST /repos/{owner}/{repo}/
   issues/{number}/comments` — a PR conversation comment IS an issue
   comment in GitHub's data model. With only `pull-requests: write`
   the call can 403 on repos whose default GITHUB_TOKEN permission
   split routes issue-comment writes through `issues:`. Adding
   `issues: write` alongside the existing `pull-requests: write` keeps
   every comment path working without expanding the trust surface
   beyond what the original validate job needed.

3. Fail-closed gate used `[ -s changed-md.txt ]` (--diff-filter=ACMRT,
   excludes deletions) but validate is now gated on `has_md_changes`
   which is computed from `[ -s diff-md.patch ]` (unfiltered, deletion-
   aware). A deletion-only PR with a missing reference DB would reach
   validate (has_md_changes=true) but bypass the fail-closed gate
   (changed-md.txt empty), masking the DB-missing condition. Switch
   the gate to `[ -s diff-md.patch ]` so both signals agree.

A separate Copilot finding (line 170, "validate gated on has_md_changes
means RST-only PRs do not run") is pushed back on the PR thread as
intentional design — RST is legacy per the RST→MyST migration, and
the RST bookkeeping in prepare exists to surface mixed-MD-RST PRs in
the Pass 2 prompt, not to drive validation on RST-only PRs.

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