fix(ci): Add missing ZPMOD_STAGE_MODULE_DIR environment variable to Docker containers

Resolves CI failure in workflow run 48420659200 where zpmod_load helper
function could not find the staged module due to missing environment variable.

Problem:
- CMake sets ZPMOD_STAGE_MODULE_DIR via test ENVIRONMENT property
- Docker containers run tests via 'ctest' but don't inherit CMake environment
- load_zpmod() function requires ZPMOD_STAGE_MODULE_DIR to prepend staging
  directory to module_path for module loading

Solution:
- Added ZPMOD_STAGE_MODULE_DIR=/workspace/build-cmake/stage/lib/zsh/site-modules
  to all test container environments in docker-compose.yml
- Updated test-minimal, test-full-dev, and test-smoke services

Verification:
- All 28 tests now pass in minimal environment (0.70s total)
- All 28 tests now pass in full-dev environment (0.70s total)
- Smoke test passes independently (0.04s total)

This completes the CI pipeline fixes started in previous commits.

Author: ss-o

docker/docker-compose.yml

@@ -9,6 +9,7 @@ services:
       - ../build-results:/workspace/build-results
     environment:
       - ZPMOD_TEST_ENV=minimal
+      - ZPMOD_STAGE_MODULE_DIR=/workspace/build-cmake/stage/lib/zsh/site-modules
       - CTEST_OUTPUT_ON_FAILURE=1
     command:
       - "sh"
@@ -26,6 +27,7 @@ services:
       - ../build-results:/workspace/build-results
     environment:
       - ZPMOD_TEST_ENV=full-dev
+      - ZPMOD_STAGE_MODULE_DIR=/workspace/build-cmake/stage/lib/zsh/site-modules
       - CTEST_OUTPUT_ON_FAILURE=1
     command:
       - "sh"
@@ -42,6 +44,7 @@ services:
     command: ["ctest", "--test-dir", "build-cmake", "-R", "zpmod_smoke", "--output-on-failure"]
     environment:
       - ZPMOD_TEST_ENV=smoke
+      - ZPMOD_STAGE_MODULE_DIR=/workspace/build-cmake/stage/lib/zsh/site-modules
       - CTEST_OUTPUT_ON_FAILURE=1
 
   # Build validation (check if source.c is properly included/excluded)