Break out validation, judging, and structural checks into lib

[dacharyc]

Summary

• Move all packages from internal/ to top-level so they're importable by external tools
• Rename validator package to skillcheck (avoids collision with go-playground/validator)
• Move main.go to cmd/skill-validator/ (standard Go project layout)
• Update build config (GoReleaser, CI) for the new entrypoint

No behavior changes. All existing tests pass.

See #16 for context and architectural decision rationale.

New install path

This is a breaking change for install path when using Go.

go install github.com/dacharyc/skill-validator@latest

Becomes:

go install github.com/dacharyc/skill-validator/cmd/skill-validator@latest

This has been a pre-release package, so I'll consider it a wash, but will bump the version to v1.0.0 and cut a release alongside some other new functionality/functionality changes.

Test plan

• go build ./cmd/skill-validator
• go test -race ./... -count=1
• golangci-lint run ./...
• Smoke test: built binary reports v1.0.0, validates testdata fixtures
• go doc ./judge confirms packages are publicly importable
• go install ./... produces correct binary name (pre-commit compat)

[dacharyc]

Closes #16