Refactor: migrate from yukifoo to Exafunction organization

[MariaChrysafis]

Refactor: migrate from yukifoo to Exafunction organization

Summary

Migrates the Claude Code Go SDK from the personal yukifoo GitHub organization to the official Exafunction organization. This includes updating module paths, import statements, documentation, and fixing a compilation issue in the examples.

Key changes:

• Updated module path from github.com/yukifoo/claude-code-sdk-go → github.com/Exafunction/claude-code-sdk-go
• Updated all import statements in example files
• Updated README.md with new badge URLs and installation instructions
• Updated LICENSE copyright attribution to Exafunction
• Added replace directive in examples/go.mod for local development
• Bug fix: Fixed field reference in quick_start.go from CostUSD → TotalCostUSD (with proper pointer handling)

The API surface remains unchanged - this maintains full compatibility with the TypeScript SDK patterns and existing user code should work with a simple module path update.

Review & Testing Checklist for Human

• Test examples compilation: Run cd examples && go build ./... to verify all examples compile with new module path
• Verify field fix: Confirm that TotalCostUSD is the correct field name in ResultMessage struct (vs the old CostUSD reference)
• Check installation: Test that go get github.com/Exafunction/claude-code-sdk-go works and verify badge URLs in README are functional

Notes

• Examples use a local replace directive (replace github.com/Exafunction/claude-code-sdk-go => ../) which is standard for monorepo development
• All tests pass and linting is clean with the new module path
• One integration test is skipped (requires Claude CLI installation)

Session info: Requested by Maria Chrysafis (@MariaChrysafis)
Link to Devin run: https://app.devin.ai/sessions/3301ee8c02fd4f2984947e9007db276f

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