fix: bump @changesets/cli to v3, required by changesets/action v2.1.2

[EmersonBraun]

What's broken

Every "Prepare Release" workflow run since PR #173 merged (00:56 UTC today) has failed:

```
This version of the Changesets action is designed to work with Changesets CLI v3.
Changesets CLI v2 is not supported; use Changesets action v1 instead, which is
compatible with CLI v2.
```

#173 bumped `changesets/action` from v1.9.0 to v2.1.2 without noticing this — the "Prepare Release" workflow only runs on push to `main`, never as a PR check, so nothing caught it during that PR's own review. Every merge since (the codeql-action bump in #283, the TypeScript 7 migration in #17, the zod v4 migration in #284) landed on `main` but never got a Version Packages PR, so none of them have reached npm — the registry is still on 0.31.0.

What's fixed

• Bumped `@changesets/cli` from `^2.31.0` to `^3.0.2` and `@changesets/changelog-github` from `^0.7.0` to `^1.0.1` — the versions `changesets/action@2.1.2` is built for. Verified locally with a real `GITHUB_TOKEN`: `changeset version` correctly consumes the three currently-pending changesets and produces a sane `0.32.0` CHANGELOG entry (reverted that local test run before committing — the real Version Packages PR should come from the automated workflow, not a hand-crafted commit).
• `release.yml`'s `changesets/action` step still used the v1 input names (`version`/`commit`/`title`). v2.1.2 silently ignores unrecognized inputs with a warning instead of failing, so our custom `version-packages` script (which regenerates `llms-full.txt` and the README Standard hash alongside the version bump) was never actually being invoked — the action was silently falling back to its own default `changeset version` only. Renamed to the v2 input names (`version-script`/`commit-message`/`pr-title`).
• Updated the one test that pinned the old `@changesets/cli`/`@changesets/changelog-github` version strings.

Testing

• Full suite: 311/311 passing.
• `npm run check` full local gate: green.
• Manually ran `GITHUB_TOKEN=$(gh auth token) npx changeset version` to confirm the CLI v3 + changelog-github v1 pairing actually produces a correct version bump and CHANGELOG entry against this repo's existing `.changeset/config.json` and changeset files — not just that the install succeeds.

Empty changeset added: this is CI/release tooling only, no package runtime behavior changes.