ci: build workspace packages before tests (round-trip fix)

[nbkdoesntknowcoding]

Follow-up to #59

PR #59 turned the Test check green through the pnpm-setup / env / migration-journal fixes, but it was merged with only its first commit — the workspace-build commit that landed on the branch afterward didn't make it into the squash. As a result the three round-trip steps still fail on main:

Error: Failed to load url @boppl/schema/node
(resolved id: @boppl/schema/node) in .../src/tests/round-trip/helpers.ts.
Does the file exist?

Root cause

The round-trip tests import @boppl/schema/node. That subpath export resolves to packages/schema/dist/node.js, which only exists after the package is built. The workflow installs deps but never builds the workspace libs, and a clean CI checkout has no dist/.

Fix (single step — tests are NOT skipped or quarantined)

Added a Build workspace packages step right after install, mirroring CONTRIBUTING step 2:

- name: Build workspace packages
  run: pnpm -r --filter "./packages/**" build

This produces packages/schema/dist/node.js (+ @boppl/shared) so vitest can resolve the import. The three round-trip steps stay gating (no continue-on-error).

Local verification

• Deleting packages/{schema,shared}/dist reproduces the exact CI failure.
• After the build step, all three round-trip steps pass, run exactly as the workflow does:

round-trip/corpus      51 passed (51)
round-trip/fuzz        30 passed (30)
round-trip/full-stack   2 passed (2)

With this, all 13 test steps in Test pass; typecheck/lint remain the only continue-on-error steps (pre-existing, unrelated — documented in #59).

🤖 Generated with Claude Code

[nbkdoesntknowcoding]

Superseded by #61, which includes this same workspace-build step plus the astro 6 + vitest 3 upgrades. Merging #61 greens the Test suite and clears the astro/vitest CVE alerts in one shot.