feat(action): add GitHub Action to verify audit CIDs (#51)

[Simultech369]

Closes #51

Summary

Implements the actions/verify-audit-cid GitHub Action for deterministic CI verification of Nirium audit trail integrity.

Key Features & Implementation

• Verifier semantics aligned with [Advanced] Standalone offline audit-CID + agent-attestation verifier (zero backend dependency) #38: Recomputes SHA-256(JSON.stringify(doc.record)), reconstructs nirium-audit-v1:<computedHash>, and never trusts embedded valid or agent.statement fields.
• Ed25519 / Stellar signer verification: Decodes Stellar G... public keys with StrKey.decodeEd25519PublicKey and verifies the embedded Ed25519 agent signature.
• Lean GitHub Action runtime: Uses standard GitHub Actions environment files for outputs and step summaries, avoiding @actions/core and its dependency surface.
• Gateway handling: Supports configurable IPFS gateways, including bare gateway URLs and /ipfs/ gateway forms.
• Pre-bundled distribution: Ships checked-in dist/index.js for zero-install workflow consumption.
• CI workflow: Adds .github/workflows/test-verify-audit-cid.yml covering action tests, live CID verification, and a deliberate tampered-fixture failure path.

Validation

• npm test -> 8/8 passing
• npm audit --omit=dev --json -> 0 vulnerabilities
• npm run package -> bundled dist/index.js
• Live CID verification via Pinata gateway -> ok=true, hashMatch=true, signatureStatus=valid
• Tamper coverage: altered record hash, forged signature, missing agent block, and statement-substitution attack