feat: implementation of ZecKit reusable GitHub Action and CI optimizations (Milestone 3)

[intelliDean]

Summary

This PR fulfils the requirements for Milestone 3 by converting ZecKit into a fully reusable GitHub Action. It enables any external repository to effortlessly spin up a Zcash devnet and execute the E2E Golden Flow (Fund → Shield → Send → Verify) against both zaino and lightwalletd backends.

(Note: The smoke-test.yml workflow will remain in a "pending/failed" state on my fork because I have reverted it to runs-on: self-hosted to match your infrastructure requirements. It will pass normally once executed on a repository with the correctly tagged self-hosted runner connected. If not revert it to runs-on: ubuntu-latest)

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Standalone Action — Flexible Funding (addresses reviewer feedback)

Inspired by the hhanh00/wallet-ci-test demo, the action now supports being used as a pure devnet-as-a-service step, where external projects supply their own addresses and run their own tests:

- uses: intelliDean/ZecKit@m3-implementation
  with:
    backend: zaino
    miner_address: ${{ vars.MINER_ADDRESS }}       # optional: direct rewards to your address
    fund_address: ${{ vars.DESTINATION_ADDRESS }}   # optional: auto-fund this wallet after startup
    fund_amount: '10.0'                             # ZEC to send (default: 10.0)
    only_setup: 'true'                              # skip ZecKit's own smoke tests
    ghcr_token: ${{ secrets.GITHUB_TOKEN }}

New CLI flags on zeckit up:

• --miner-address — override Zebra's mining reward address (instead of embedded default)
• --fund-address — auto-fund a destination address once the devnet is healthy
• --fund-amount — amount in ZEC to send (default 10.0)

A full working example is in the sample project.

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Key Changes

GitHub Action Integration (action.yml)

• Created a standardised composite action for external consumption.
• Hardened GHCR container caching by injecting strict --profile flags during docker pull, drastically reducing workflow execution time from 20+ minutes down to seconds.
• Added deterministic JSON metric logging (run-summary.json and faucet-stats.json) to allow external repositories to make assertions on transaction data.
• Secured artifact generation by dynamically scoping zip names to backend matrices to prevent parallel upload collisions.
• Added miner_address, fund_address, fund_amount, and only_setup inputs for standalone usage.

CLI Workflow Generator (zeckit init)
Automatic CI Setup: Introduced the zeckit init command, which automatically generates a production-ready GitHub Actions workflow file (.github/workflows/zeckit.yml).
Configurable Backends: Supports generating workflows for both zaino and lightwalletd backends via the --backend flag.
Developer Experience: Simplifies the integration of ZecKit into external repositories by providing a one-command solution for CI bootstrapping.

CLI & Automation Enhancements (test.rs)

• Integrated a continuous background miner into the zeckit test command sequence.
• Implemented fail-fast registry diagnostics by removing silent Docker fallback mechanisms.

Documentation & Workflows

• Updated README.md with explicit configuration instructions.
• Directed smoke-test.yml to match maintainer server configurations.