feat: add staging environment infrastructure including CI pipeline, c…

[akinboyewaSamson]

closes #143

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## Description
This PR establishes a dedicated staging environment workflow, configuration setup, and automated health checking mechanism for NotifyChain. This allows contributors to validate on-chain contract interactions and off-chain notification delivery mechanics in a unified environment prior to main branch integration.

## Key Changes
* **Deployment Workflow:** Added `.github/workflows/staging.yml` to automate dependency installation and builds for both the `listener` service and the React `dashboard` upon pushes to the `staging` branch.
* **Environment Orchestration:** Created dedicated `.env.staging` profiles for both the listener subsystem (targeting Stellar Testnet RPC) and the frontend application dashboard.
* **Automated Health Check:** Implemented a lightweight shell utility (`scripts/health-check.sh`) integrated directly into the CI pipeline to ping and validate the listener's `/health` endpoint post-build.
* **Documentation:** Appended local staging validation instructions to the root `README.md`.

## Verification Tasks Completed
* [x] Staging deployment pipeline configurations validated.
* [x] Environment variable templates created.
* [x] Target health status shell script marked executable and verified.