feat: improve CLI usability

[im10furry]

Summary

• Let scion add <module> and scion diff <module> use registry default targets when --to or --target is omitted.
• Add command-specific help via scion help <command> and --help/-h on subcommands.
• Make scion info auth clearly show the standalone requirement and suggested add command.
• Support common flag ordering such as scion add --to internal/cache --dry-run cache.
• Update README and getting-started docs for the shorter CLI workflow and v0.1.3 examples.

Verification

• go test -count=1 ./...
• go vet ./...
• go run ./cmd/scion doctor --strict
• go run ./internal/cmd/build-bundle; git diff --exit-code -- internal/bundle
• cd docs && npm run build
• Local smoke with built CLI: add cache, diff cache, help add, info auth