feat: ZecKit GitHub Action, golden E2E workflow, GHCR image publishing

[Great-DoA]

Summary

Introduces ZecKit as a reusable action.yml publishable to the GitHub Marketplace,
along with the CI infrastructure to build and share pre-built container images and a golden E2E workflow.

Changes

GitHub Action (action.yml)

• New composite action "ZecKit E2E" — spins up the full Zcash devnet using
  pre-built GHCR images and runs the complete golden shielded-transaction flow:
  Generate UA → Fund → Autoshield → Shielded Send → Rescan/Sync → Verify
• Supports backend input (zaino / lwd), configurable timeouts, optional
  fresh-start, and produces structured log artifacts
• Designed for use as - uses: zecdev/ZecKit@v1 from any external repo

New Workflows

• build-images.yml — builds and publishes all container images to GHCR
  (ghcr.io/<owner>/zeckit/*) on push and workflow_dispatch
• ci-action-test.yml — exercises the composite action itself in CI,
  covering both backends in a matrix
• golden-e2e.yml — standalone golden E2E workflow callable via
  workflow_call and workflow_dispatch

Updated Workflows

• e2e-test.yml — migrated from self-hosted to ubuntu-latest; added
  GHCR authentication, pre-built image tag selection (SHA → branch → latest
  fallback), and explicit permissions block
• smoke-test.yml — aligned to match the new runner and image-prefix
  conventions