feat(opf): ship examples, docs, and upstream README in the npm package

[michaeljgrimm]

Adds three new subpath exports for downstream sites that currently have to clone OpenPresentation/opf or hit GitHub at request time to read content the package doesnt yet ship.

• @openpresentation/opf/examples bundles every .opf.json under examples/ at build time, exposes them as ExampleRecord[] + GalleryRecord[] (with the parsed Presentation deck), plus convenience lookups: getExample, getGallery, getExamplesByGallery, getExamplesByCategory. Each deck is validated against the presentation schema during smoke tests.
• @openpresentation/opf/docs bundles the top-level docs/*.md reference pages (schema-reference, catalog-schema-reference, content-payloads, content-item-design-overrides, examples) and exposes them as DocRecord[] with getDoc. The BACKLOG and docs/migrations/, docs/plans/ subdirectories are intentionally excluded because they change too quickly to pin inside a release.
• @openpresentation/opf/repo-readme exposes the upstream README.md as a raw markdown string so consumer sites can mirror the canonical README without a network fetch.

Implementation:

• New scripts/generate-content.mjs walks examples/, docs/, and README.md at build time and emits src/generated/{examples,docs,repo-readme}.ts; wired into the generate / build pipeline alongside the existing schema, catalog, spec-files, and previews generators.
• New facade modules src/examples.ts, src/docs.ts, src/repo-readme.ts re-export the generated data with the public type surface.
• tsup config gains three entries; package.json gets three new exports.
• Smoke test asserts every shipped deck validates, that gallery indexing is consistent, that docs metadata is non-empty, and that the repo README is present and non-trivial.
• README and CHANGELOG document the new entry points.

Pack stays at one tarball; size grows from ~1 MB to ~1.7 MB (unpacked 13.5 MB) which is acceptable for the convenience of removing the GitHub source sync from downstream consumers.