chore: reduce repository root entries

[linkdata]

Summary

• reduce tracked repository-root entries from 66 to 39 without changing any of the 16 Go package import paths
• consolidate root production sources, error definitions, and tests by subsystem, reducing root test files from 28 to 10
• move the security audit and request-reuse exercise script under .github, updating their references and repository-root resolution

Verification

• go generate ./...
• go vet ./...
• gofmt -l .
• staticcheck ./...
• golangci-lint run
• gosec ./...
• go build ./...
• JAWS_REQUIRE_NODE=1 go test -race ./...
• JAWS_REQUIRE_NODE=1 go test ./...
• declaration-level verification preserved every moved implementation and all 395 test, fuzz, and benchmark entry points
• temporary Go workspace resolved ../xyzzy to this checkout; go build -v ./..., go test -race ./..., and go test ./... passed there