docs: add review/release receipt layer design note

[SemeAIPletinnya]

Motivation

• The runtime/API now exposes structured review evidence for NEEDS_REVIEW decisions and the project needs a conservative, documented design direction for turning that evidence into auditable receipts later.
• This note clarifies boundaries so future work can add a stable receipt schema without conflating runtime behavior, policy authority, or safety/compliance claims.

Description

• Add a new design note at docs/review_release_receipt_layer.md that documents the core idea, current available review-evidence fields, a proposed future receipt schema, illustrative example, receipt types, relationship to existing artifacts, limitations, and next steps.
• Link the new design note from the docs index at docs/README.md, add it to the project navigation map at docs/project_navigation.md, and add a cross-reference from the review evidence section in docs/runtime_decision_contract.md.
• Changes are documentation-only and explicitly state they do not change runtime code, API schemas, release policy, replay logic, benchmark metrics, tests, provider integrations, or make safety/compliance claims.

Testing

• Ran git diff --check and the tree-level check was clean.
• Ran the full test suite with python -m pytest -q --basetemp=... which completed with 281 passed, 1 warning.
• The change set touches documentation files only and adds docs/review_release_receipt_layer.md alongside updated references in docs/README.md, docs/project_navigation.md, and docs/runtime_decision_contract.md.

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Codex Task