feat(sdk): s2s create --library, worked examples, docs

Adds the library-authoring on-ramp closing out the library-packages slice:
CreateOptions.kind + the --library flag scaffold a library's package.json
(s2script.kind: "library", main/types at src/index.ts/.d.ts, no
pluginDependencies) and a real exported function + its .d.ts, with the
interactive wizard offering the choice; refused inside workspace mode for
now since a workspace library needs to sit outside the plugins/ glob a
member's scaffold writes into.

examples/library-package (@example/base64, a hand-written base64 codec with
deliberately no Buffer/atob/btoa) + examples/library-consumer (calls
encode/decode via a committed .s2script/libs/ vendored tree, produced by
building and extracting rather than hand-writing) exercise the
bundle-and-run path end to end; verified the library's code is genuinely
inlined into plugin.js, not left as an external require().

docs/ARCHITECTURE.md gets a package-format subsection on the two kinds,
.s2lib, and why libraries are vendored rather than npm-installed; README.md
and packages/sdk/README.md each get a line pointing at s2s add. Revises the
existing .changeset/library-packages.md prose to cover the finished
create/add surface rather than adding a second changeset.