Add GitHub Copilot instructions for Postman.gov.sg development workflow

[Copilot]

This PR adds comprehensive GitHub Copilot instructions to help the coding agent work effectively in the Postman.gov.sg codebase. The instructions provide validated commands, timing expectations, and troubleshooting guidance for common development tasks.

Key Features

Environment Setup & Prerequisites

• Node.js 18.15.0 requirement (from .nvmrc)
• Required tools installation (NVM, detect-secrets)
• Environment variable configuration with working examples

Validated Build & Development Workflow

• Complete dependency installation process with Sentry CLI workarounds
• Build commands with measured timings:
  • Full build: 2-3 minutes (NEVER CANCEL)
  • Individual components: 12-60 seconds each
• Database setup with Docker services and migrations

Testing & Quality Assurance

• Test suite execution with timings:
  • Backend tests: ~2 minutes
  • Shared tests: <10 seconds
• Linting and precommit workflows
• End-to-end validation scenarios

Critical Timeout Guidelines
All long-running operations include explicit "NEVER CANCEL" warnings and timeout recommendations:

• npm install: 180+ seconds
• npm run build: 300+ seconds
• npm test: 300+ seconds
• Database operations: 60+ seconds

Common Issues & Solutions

• Dependency installation failures (Sentry CLI, legacy peer deps)
• Frontend build issues in CI environments (CI=false workaround)
• Database connection and migration problems
• TypeScript compilation dependencies

Validation Requirements
The instructions emphasize thorough validation after code changes:

• Build verification with timeout specifications
• API connectivity testing (validates 401 responses at localhost:4000/v1/campaigns)
• Database connectivity through migration runs
• Manual end-to-end workflow verification

All commands and timings were validated through actual execution in the development environment to ensure accuracy and reliability.

Fixes #2305.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• downloads.sentry-cdn.com
  • Triggering command: node scripts/install.js (dns block)
• iojs.org
  • Triggering command: curl --fail --compressed -q -L -s REDACTED -o - (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.