[codex] Add DeltaBox checkpoint event schema

[yangfei222666-9]

Summary

• Add a docs-only DeltaBox checkpoint/rollback event schema draft.
• Map checkpoint and rollback semantics to Product Spine gates: Boot Preflight, EventFlow, Scope Isolation, Artifact Memory, and Closeout.
• Preserve explicit non-claims for runtime checkpointing, provider readiness, judgment, promotion, paper-buy, trade, and repo-wide PASS.

Verification

• Confirmed latest local commit before push: b2dee06 docs: add DeltaBox checkpoint event schema.
• Parsed runs/ops_check/deltabox_checkpoint_event_schema_draft_20260523/summary.json.
• Parsed runs/ops_check/deltabox_checkpoint_event_schema_draft_20260523/event_flow.jsonl with event_count=9.
• Confirmed unsafe_boundary_flags=[].

Boundaries

• Docs-only / learning-only.
• No runtime checkpointing implemented.
• No secret read, broker/provider connection, judgment, promotion, paper-buy, trade, or pass-to-trade.
• Outside dirty buckets remain excluded from this PR.