PR 3/N: publish as @textcortex/slidewise via changesets + CI

[karthikmudunuri]

Third PR in the integration plan. Stacked on top of #6 — review/merge that first (which is on #5, on #4).

Summary

Wires Slidewise up to be released as @textcortex/slidewise via changesets and GitHub Actions, so platform hosts can pin a real semver range instead of cross-repo workspace deps.

What's added

• package.json — scoped to @textcortex/slidewise, publishConfig (access: restricted, default registry npmjs.org), repo/homepage/bugs metadata, changeset / version-packages / release scripts. React and react-dom moved from dependencies to devDependencies — peerDependencies still declares the runtime contract (>=19), so consumers don't end up with a duplicate React.
• .changeset/ — initialised by changesets init. Default config matches our publish settings. First changeset (initial-1-0-0.md) marks v1.0.0 as the public API contract.
• .github/workflows/ci.yml — typecheck + vitest + build:lib on every PR and push to main.
• .github/workflows/release.yml — on push to main, runs the same checks then changesets/action. With pending changesets present it opens (or updates) a "Version Packages" PR; once that PR merges it publishes to the registry. Skips publish if NPM_TOKEN is unset.
• README.md — quick-start with the real SlidewiseFileEditor API (loadBlob/saveBlob), lower-level SlidewiseEditor example, programmatic parsePptx/serializeDeck/migrate snippet, release docs, repo layout.
• src/index.ts — re-exports migrate and CURRENT_DECK_VERSION so the README's persistence snippet compiles for consumers.

Release flow once merged

1. Merge this PR.
2. CI on main sees a pending changeset; opens a "Version Packages" PR bumping 0.2.0 → 1.0.0 and writing CHANGELOG entries.
3. Maintainer reviews and merges the Version Packages PR.
4. CI runs again, finds no pending changesets, runs changeset publish → first release of @textcortex/slidewise@1.0.0 to the registry.

Open items for the platform team

These don't block merging this PR but they gate the first actual publish:

• Registry decision: keep npmjs.org with a paid @textcortex scope, or point publishConfig.registry at an internal Verdaccio / GitHub Packages npm registry.
• NPM_TOKEN secret: add an automation token with publish rights for the @textcortex scope (or whatever registry the team picks) to repo Actions secrets.
• Branch protection: require the CI / Typecheck, test, build check on main once the workflow has run once.

Test plan

• pnpm exec tsc -b clean
• pnpm test — 14/14
• pnpm run build:lib — emits dist/slidewise.css (159 kB) + dist/index.mjs (864 kB), no errors
• pnpm exec changeset status — reports the pending v1.0.0 major bump
• Verify CI workflow runs green once the workflow file exists on main
• Platform team confirms registry + adds NPM_TOKEN