fix(delete): block self-approval of protected-page deletes

[dalledajay-coder]

What changed

_approval_block_reason now applies the protected-page-kind gate to DELETE proposals that target a page, not just to PAGE proposals. the page kind is resolved from the live artifact (what approve would actually remove), falling back to the propose-time snapshot so the guard also holds on the idempotent already-gone path. check_approvable shares the helper, so the batch precheck reports the same reason. unprotected deletes keep the trusted-agent opt-out unchanged.

Why

with review.approver_role: trusted-agent and a protected page kind (say voice), editing that page correctly refuses self-approval — "it always requires a reviewer other than the proposer". but the new delete path didn't run that check, so the same proposer could file kb.propose_delete for the page and approve it themselves:

edit_pr = propose_page(store, title="email voice", body="b", page_type="voice", proposed_by="agent")
approve(store, edit_pr.id, approved_by="agent")   # ProposalError: protected
del_pr = propose_delete(store, target_kind="page", target_id="email-voice", proposed_by="agent")
approve(store, del_pr.id, approved_by="agent")    # succeeded — page gone

removing a policy-bearing page is at least as sensitive as editing it, and the comment on the existing gate says the opt-out "can never widen it" — the delete path widened it. deletes are also irreversible (reversible=False in the audit event), so this was the one write a trusted agent could make to protected knowledge without a second reviewer.

no .vouch/ directory migration concerns: the change only tightens an approve-time check; nothing on disk changes shape.

Testing

• tests/test_delete.py::test_delete_protected_page_blocks_self_approval_despite_trusted_agent — fails on current test (the approve succeeds and the page is unlinked), passes with the fix; also asserts a distinct reviewer can still approve.
• tests/test_delete.py::test_delete_protected_page_guard_holds_when_target_already_gone — covers the snapshot fallback on the crash-retry path; fails before, passes after.
• tests/test_delete.py::test_delete_unprotected_page_keeps_trusted_agent_opt_out — guards against over-blocking; passes before and after.

ran locally:

python -m ruff check src tests   # all checks passed
python -m mypy src               # success: no issues found in 91 source files
python -m pytest tests/ -q --ignore=tests/embeddings   # 1153 passed, 28 skipped

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