ci(export): add e2e packed export build verification to pr checks

[pasevin]

Summary

• Add a new e2e-export job to the Export Testing workflow that runs the full packed export pipeline (--env packed) on every PR, catching type/bundling/CSS regressions before merge
• Broaden the workflow trigger from export-path-only to all PRs, since adapter, type, style, and plugin changes can break exports (as demonstrated by the recent adapter export, dts.resolve, and global.css regressions)

What the e2e job does

1. Builds all monorepo packages
2. Packs adapter packages as tarballs (simulating npm publish)
3. Generates a form export via the Vitest wrapper
4. Replaces dependencies with file: tarball paths
5. Runs pnpm install && pnpm build on the exported app (verifying tsc + vite build + Tailwind resolution)

If any step fails, the PR check fails.

Two parallel jobs

• Test Form Exports (existing, fast) — unit tests for export logic
• E2E Export Build Verification (new, ~4-5 min) — full packed export + build

Test plan

• Verify both jobs appear in the PR checks
• Verify the e2e-export job passes on this PR (it exercises the current main codebase)
• Verify a deliberate break (e.g., removing an adapter export) would fail the check