[P1] Publication guard: PKCS#8 headers, shared exporter set, prepare_publication CI wiring

[stranske]

Why (verified evidence)

Post-merge verification (D4-verify-merged-2026-09-16T02) of #574 (merge e97c029f5a3e983e1838dec636fd773ed04f232b) against #554 repaired the research-local allowlist location, but several adversarial publication-safety items from the issue remain unmet in the squash diff.

• scripts/check_publication_safety.py / tests/test_publication_safety.py expanded, but PKCS#8 and ENCRYPTED PRIVATE KEY PEM headers are not covered in test_credential_patterns / test_all_private_key_formats_are_rejected (diff shows RSA/OPENSSH only).
• No shared header constant wired between scanner and scripts/prepare_publication.py exporter in the diff.
• prepare_publication.py is documented in README but not wired into .github/workflows/publication-guard.yml (workflow unchanged in diff).

Tasks

• Add PKCS#8 and ENCRYPTED PRIVATE KEY detection to scripts/check_publication_safety.py (shared with exporter if applicable).
• Extract one shared private-key header set used by both check_publication_safety.py and scripts/prepare_publication.py.
• Add one pytest per key format in tests/test_publication_safety.py.
• Wire scripts/prepare_publication.py into the publication guard workflow so preparation failures block merge.

Acceptance Criteria

• Named test: pytest tests/test_publication_safety.py -k private_key exits 0 with PKCS#8 and ENCRYPTED fixtures rejected.
• Deliberate-break → revert: remove PKCS#8 header from scanner set → test_all_private_key_formats_are_rejected FAILS → restore → passes.

Non-Goals

• Do NOT revert the research-local allowlist path fix from fix(publication): honor research-local allowlist policy #574.
• No scaffolding / TODO-only changes.

Surfaced by D4-verify-merged-2026-09-16T02; verified against squash diff Ready-574.diff and issue #554. Related: #554, merged PR #574.