ci: changesets and GitHub Actions release pipeline

[gabeklein]

Follow-up to #2 - the changesets + CI/CD half of the bun migration, mirroring expressive-mvc's setup.

build: configure changesets with github changelog

• Replaces the stale pnpm-era .changeset/config.json (default changelog, access: restricted - which would have broken publishing scoped packages) with mvc's config: GitHub-linked changelog via changelog-no-thanks.cjs, access: public.
• Adds a changeset for the bun migration itself: minor for @expressive/sqlite (pluggable driver), patch for sql/postgres/orm.
• Root package.json: packageManager: bun@1.3.14, stricter engines.

ci: bun-based PR verification and OIDC release workflows

• .github/actions/setup: composite action (setup-bun, dependency cache, frozen-lockfile install).
• pr.yml: blocking bun run test + bun run build; non-blocking changeset-status preview and npm publish --dry-run pack validation for all four packages.
• release.yml: replaces the dead (if: false) pnpm workflow. changesets/action maintains the Version Packages PR and publishes via npm OIDC trusted publishing - no token secrets. See .github/workflows/README.md.

Setup required before merging (GitHub/npm side)

• Create a release environment on this repo, deployment branches restricted to main.
• On npm, add this repo's release.yml (environment release) as a trusted publisher for each of the four packages.

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