[ENHANCEMENT] Update CI to use sliced coverage and add merged-report guard #118

Problem (one or two sentences)

CI currently runs pnpm test:coverage (a single monolithic task); after slicing, it must run test:coverage:merge to trigger all slices and produce the merged report, and needs a guard to prevent silently uploading an empty or missing report to Codecov.

Context (who is affected and when)

The Codecov upload step in .github/workflows/code-qa.yml:74 expects src/coverage/lcov.info. This path is produced by the merge task added in #117. The upload step itself does not need to change — only the step that generates the file. This is the final step in the coverage-slicing work tracked in #47 and depends on #117.

Desired behavior (conceptual, not technical)

CI runs pnpm turbo run test:coverage:merge --filter=zoo-code instead of pnpm test:coverage. A post-step validates the merged report is present and non-empty before upload. On a commit touching only src/api/, CI logs show cache hits for the four unaffected slices.

Constraints / preferences (optional)

• The Codecov upload path (src/coverage/lcov.info) must not change.
• Baseline to beat: ~8m24s for the full coverage step on an Ubuntu runner (from CI run 25606102845).
• Record new CI runtime and cache-hit rates in the PR notes as evidence.

Request checklist

• I've searched existing Issues and Discussions for duplicates
• This describes a specific problem with clear context and impact

Acceptance criteria (optional)

• CI passes end-to-end.
• A commit touching only src/api/ shows cache hits for test:coverage:core, test:coverage:services, test:coverage:misc, and test:coverage:dist.
• lcov --summary src/coverage/lcov.info exits 0 and reports non-zero covered lines.

Proposed approach (optional)

1. Replace run: pnpm test:coverage with run: pnpm turbo run test:coverage:merge --filter=zoo-code in code-qa.yml.
2. Add a step: run: lcov --summary src/coverage/lcov.info (exits non-zero if file is missing or has 0 covered lines).
3. Verify the Codecov upload step is unchanged.

Trade-offs / risks (optional)

• lcov may not be pre-installed on the GitHub Actions Ubuntu runner; add an install step if needed.
• The first CI run after merging will be a full cache miss; subsequent runs will show partial hits.

Parent issue: #47
Depends on: #117