[ENHANCEMENT] Add per-area Turbo coverage tasks and a coverage merge task

[edelauna]

Problem (one or two sentences)

No Turbo tasks exist for the individual coverage slices, so they cannot be independently cached or scheduled, and there is no way to produce the single coverage/lcov.info that CI expects from partial runs.

Context (who is affected and when)

CI uploads src/coverage/lcov.info to Codecov (.github/workflows/code-qa.yml:74). This path must remain stable after slicing. The lcov -a flag merges multiple tracefile inputs into a single output — the standard approach for combining V8+lcov slice outputs. This is part of the coverage-slicing work tracked in #47 and depends on #115 and #116.

Desired behavior (conceptual, not technical)

Five Turbo tasks (test:coverage:api, test:coverage:core, test:coverage:services, test:coverage:misc, test:coverage:dist) exist in src/turbo.json, each with isolated outputs. A test:coverage:merge task depends on all five and produces the merged coverage/lcov.info.

Constraints / preferences (optional)

• Each slice task should only declare dependsOn: ["bundle"] at the package level; root-level @roo-code/types#build is inherited.
• The merge script must produce output at exactly coverage/lcov.info to match the existing CI upload path.
• Verify lcov is available in the CI environment before merging; consider lcov-result-merger (npm) as a fallback.

Request checklist

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

Acceptance criteria (optional)

• pnpm turbo run test:coverage:merge --filter=zoo-code produces coverage/lcov.info whose total line coverage matches the current full-run output within a reasonable margin.
• A commit touching only src/api/ shows Turbo cache hits for test:coverage:core, test:coverage:services, test:coverage:misc, and test:coverage:dist.

Proposed approach (optional)

1. Add five test:coverage:{area} tasks to src/turbo.json, each with dependsOn: ["bundle"] and outputs: ["coverage/{area}/**"].
2. Add a test:coverage:merge task (or script) running:

   lcov -a coverage/api/lcov.info -a coverage/core/lcov.info -a coverage/services/lcov.info -a coverage/misc/lcov.info -a coverage/dist/lcov.info -o coverage/lcov.info
   

3. test:coverage:merge depends on all five slice tasks and outputs: ["coverage/lcov.info"].

Trade-offs / risks (optional)

If lcov is not installed in the CI environment, the merge step will fail. Verify availability or use an npm alternative.

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Parent issue: #47
Depends on: #115, #116

[zoomote]

Current main after #1614 supersedes the proposed five-slice merge design. Profiling and coverage equivalence showed test:coverage:unit already owns the complete extension source report (467 files and 30,227 instrumented lines), while test:dist only validates emitted bundle assets and contributes no source coverage. A merge task would therefore add tooling and cache dependencies without adding coverage. Draft PR #1620 wires those measured lanes directly into CI and references this issue without claiming to implement its stale five-way acceptance criteria.