ci: add windows-latest to test matrix

[clean6378-max-it]

closes #61

Summary

Add windows-latest to the CI test matrix so pytest, API integration tests, and Vitest run on both Ubuntu and Windows. Adds Windows-native tests for export-state locking (msvcrt.locking) and Claude projects path resolution (USERPROFILE).

Closes Chen June Week 1 sprint item #2 (5 pt).

Problem

CI previously ran only on ubuntu-latest. Two code paths differ on Windows but were never exercised on a real Windows runner:

• utils/export_state_store.py — msvcrt.locking (existing tests only mocked msvcrt on Linux)
• utils/session_path.py — get_claude_projects_dir() uses USERPROFILE when platform.system() == "Windows"

Changes

• .github/workflows/ci.yml

  • Matrix os: [ubuntu-latest, windows-latest] with fail-fast: false for pytest, integration-tests, and js-tests
  • Coverage artifacts named coverage-report-${{ matrix.os }}
  • mypy and prod-install-smoke unchanged (Ubuntu only)

• tests/test_export_state_store.py

  • test_export_state_lock_real_msvcrt_roundtrip — runs on win32 only; uses real msvcrt via export_state_lock + atomic_write_export_state

• tests/test_session_path.py (new)

  • test_get_claude_projects_dir_uses_userprofile_on_windows — mocked Windows + USERPROFILE
  • test_get_claude_projects_dir_on_windows_runner — native Windows runner

• CONTRIBUTING.md

  • Documents Ubuntu + Windows CI matrix

Test plan

• pytest -q locally on Windows (348 passed)
• npm test locally on Windows (39 passed)
• GitHub Actions: pytest × 2 OS green
• GitHub Actions: integration-tests × 2 OS green
• GitHub Actions: js-tests × 2 OS green
• GitHub Actions: mypy + prod-install-smoke still green on Ubuntu

Out of scope

• macOS runners
• export_count deprecation (Thursday #4)
• the-claw benchmarks (Thursday UI/UX design overhaul: theme, animations, interactive components (#5) #7)
• Locking implementation changes (unless CI exposes a bug)

Summary by CodeRabbit

• Tests

  • Added Windows-specific tests for export-state locking and session-path handling to improve Windows compatibility and concurrency guarantees.

• Chores

  • CI matrix expanded to run pytest, integration tests, and JS tests on both Ubuntu and Windows; per-OS coverage artifacts produced and matrix fail-fast disabled.

• Documentation

  • CONTRIBUTING guide and PR checklist updated to reflect the expanded CI matrix and job expectations.