chore: rename GitHub org from onkernel to kernel

[hiroTamada]

Summary

Update the codebase to reflect the GitHub organization rename from onkernel to kernel.

• Update go.mod module path from github.com/onkernel/hypeman-go to github.com/kernel/hypeman-go
• Update all Go source file import paths (56 files)
• Update documentation references in README.md, CONTRIBUTING.md, CHANGELOG.md, and api.md

Verification

• ✅ go build ./... - All packages compile successfully
• ✅ ./scripts/test - All tests pass (mock server uses kernel/hypeman OpenAPI spec)
• ✅ ./scripts/lint - No lint errors

Test plan

• Verify Go build succeeds
• Verify all tests pass
• Verify Stainless integration works (test output shows OpenAPI spec from kernel/hypeman)

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Note

Migrates repository references from github.com/onkernel/hypeman-go to github.com/kernel/hypeman-go.

• Update module path in go.mod and all Go imports (source, tests, lib/, examples/)
• Refresh docs and pkg links in README.md, CONTRIBUTING.md, CHANGELOG.md, and api.md (badges, compare URLs, issue links)
• No functional code changes; build/tests continue to use the new module path

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