ci: add API Extractor configuration and automated public API surface …

[Oluwasuyi-Oluwatimilehin-Daniel]

#closes #91

📝 Description

Resolves #91

This PR implements an automated Public API Report system using @microsoft/api-extractor. It establishes a monitored baseline for our exported classes, configurations, types, and interfaces, ensuring accidental breaking changes or unintended surface exposure are caught before they hit production.

⚙️ Changes Introduced

• Dependency Management: Added @microsoft/api-extractor to devDependencies.
• Configuration: Created api-extractor.json targeting dist/index.d.ts as the primary API surface entry point.
• Automation Scripts: Added pnpm api-report (local updating) and pnpm api-report:ci (strict verification) to package.json.
• CI/CD Guardrail: Set up .github/workflows/ci.yml from scratch to enforce API snapshot matching on all incoming PRs.
• Documentation: Updated CONTRIBUTING.md with operational guidance for contributors who modify public type signatures.
• Baseline Report: Generated the initial public API snapshot in api-report/guildpass-sdk.api.md.

🧪 Verification Results

Successfully generated the baseline report locally via Power温shell:

Analysis will use the bundled TypeScript version 5.9.3
Generating complete API report: .../api-report/guildpass-sdk.api.md
Warning: The API report file was missing, so a new file was created.
API Extractor completed successfully
#closes