feat: add SNIP-12 signer mode for Dfns-backed session signing (v2, feature-flagged)

[omarespejel]

Parent: keep-starknet-strange/starknet-agentic#254

Problem

SISNA currently attempts Dfns kind=Hash for session signing. On Starknet keys this fails (invalid signature kind).

Scope

Implement SNIP-12 signing mode in SISNA backend while preserving current mode for fallback.

Deliverables

• Add signer mode: session_signature_mode=v2_snip12 (config + docs)
• Build canonical SNIP-12 typed payload from session request
• Call Dfns with kind=Snip12
• Verify returned signature locally against configured verification key
• Keep v1 path operational behind flag
• Structured logs include signature_mode, request_id, message_hash/domain_hash

Test requirements

• Unit tests: canonicalization, domain, chainId normalization, field ordering
• Integration tests: Dfns adapter mocked + failure modes
• Backward-compat tests for existing session flow

Security requirements

• Replay protections unchanged
• Policy checks unchanged (selector deny, account allow, key allow)
• Explicit fail-closed behavior on unsupported signature kinds

Definition of done

• npm test green
• npm run build green
• Demo artifact generated for v2 mode

[omarespejel]

Observed with live Dfns Starknet key probe (2026-02-15):\n- SignatureKind.Hash => Failed (reason: invalid signature kind)\n- SignatureKind.Message => unsupported on Starknet\n- SignatureKind.Transaction => unsupported on Starknet\n- SignatureKind.Snip12 => Signed\n\nThis issue is P0 because current Dfns integration cannot complete session signing on the raw-hash path.