refactor: expose explorer as an embeddable library package

[phsym]

What

Move the TUI out of internal/ into the module-root explorer package so it can be embedded into other applications, and relocate the CLI entrypoint to cmd/kmip-explorer.

• internal/explorer.go → explorer.go (package internal → explorer)
• rename NewExplorer → New; document the public API (New, Run, Explorer)
• main.go → cmd/kmip-explorer/main.go, importing the explorer package
• update goreleaser build dir and CI build path to ./cmd/kmip-explorer
• update the go install path and add a library-usage section to the README

Breaking change

Source installs must now use:

go install github.com/phsym/kmip-explorer/cmd/kmip-explorer@latest

Binary releases are unaffected — the release binary name stays kmip-explorer.

Verification

• go build ./..., go vet ./... pass; gofmt -l clean
• all references to the old internal/NewExplorer paths updated (CLI, goreleaser, CI)