Draft approval signing design

[SSBrouhard]

Summary

• add docs/approval_signing_design.md as the v0.3.0 design gate for asymmetric, out-of-domain approval signatures (Asymmetric out-of-domain approval signatures #24)
• define the key-isolation boundary, signed payload, public-key handling, fail-closed verification states, compatibility modes, CLI sketch, and acceptance gates
• link approval signing to the evidence integrity boundary from Add tamper-evident evidence ledger and verification command #2 without changing runtime behavior
• add a minimal pointer from docs/evidence.md while preserving current HMAC/advisory wording

Follow-up issues

• Asymmetric out-of-domain approval signatures #24 Asymmetric out-of-domain approval signatures
• Action-tier catalog doc #25 Action-tier catalog doc
• Multi-turn pressure evidence #26 Multi-turn pressure evidence

Validation

• .venv/bin/python scripts/vmga_release_check.py
• .venv/bin/python -m compileall src tests scripts integrations
• .venv/bin/python -m pytest -q
• git diff --check
• forbidden-vocabulary scan for the sourcebook terms called out in the task

Design/docs only. No crypto code and no behavior change.