[FEAT] Add unit tests, upload flag, CI action, and JSON docs (#43)

[IISweetHeartII]

Description

P1 post-hackathon improvements for ax-score: comprehensive test suite, upload capability, CI action template, and JSON output documentation.

Type of Change

• New feature (non-breaking change which adds functionality)

Changes Made

• Unit tests (128 tests across 25 files): Full test coverage for all 19 audits, 3 gatherers, scoring engine, runner pipeline, and upload module using vitest with mocked HTTP responses
• --upload CLI flag: New -u, --upload flag with --api-url and --api-key options to send scan results to the AgentGram hosted API (POST /api/v1/ax-score/scan). Reads AGENTGRAM_API_KEY env var as fallback. Handles auth errors, timeouts, and network failures gracefully
• uploadReport() library function: Exported from the main package for programmatic use
• GitHub Action template (.github/actions/ax-score/action.yml): Composite action that installs ax-score, runs an audit, optionally uploads results, and fails if the score is below a configurable threshold. Outputs score and report for downstream steps
• JSON output contract docs (docs/json-output-contract.md): Complete schema documentation for the AXReport type returned by runAudit() and --format json, with field descriptions and a full example
• CI integration guide (docs/ci-integration.md): Usage examples for the GitHub Action including basic, threshold, upload, and output configurations
• Version bump: 0.2.0 -> 0.3.0

Related Issues

Closes #43

Testing

• Manual testing performed
• Type check passes (pnpm type-check)
• Lint passes (pnpm lint)
• Build succeeds (pnpm build)
• All 128 tests pass (pnpm test)

Checklist

• My code follows the project's code style
• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• My changes generate no new warnings