Add .pubignore to exclude development-only content from the published
package archive — upstream ACP spec reference copies under docs/, the
marketing website/, codegen tooling and bundled schemas under tool/, and
historical planning docs under doc/plans/. Compressed archive drops from
~471 KB to ~129 KB.

Fix README installation snippet to pin to 0.1.0-rc.1 (caret ranges don't
include pre-releases in pub, so the old ^0.1.0 wouldn't resolve).