docs: add test-first contribution guide

[Carlys17]

Closes #114

Description

Add a new documentation file docs/test-first-contribution-guide.md that defines the test-first contribution methodology for the Aegis SDK.

The guide covers:

• When tests are required (table of change types)
• Module-specific test expectations for all six areas: compliance checks, RWA metadata, admin actions, investor reads, transaction receipts, and typed SDK errors
• Happy-path and negative-path requirements for every test file
• No-test justification rules (valid and invalid examples)
• Local test commands
• Mock client vs RPC mocks guidance
• Test file checklist

The README is updated with a link to the new guide under the Contributing section.

Acceptance Criteria Coverage

• Test-first contribution guide is added
• Testing expectations for compliance checks, RWA metadata, admin actions, investor reads, transaction receipts, and typed SDK errors are documented
• Happy-path and negative-path test expectations are included
• No-test justification rules are documented
• Local test commands are included
• README links to the guide