feat: make module setup one click and add PR CI

[wellwei]

Summary

• replace the multi-step module setup wizard with one modal confirmation followed by locked LLVM install/default/build and automatic clangd configuration
• refuse project-pinned non-LLVM toolchains without editing mcpp.toml, reload the LLVM CDB after a forced rebuild, and preserve degraded build reporting
• add deterministic VS Code Extension Host smoke coverage with fake mcpp/clangd fixtures and separate PR unit/package and E2E CI jobs

Closes #6
Closes #7

Test plan

• npm test (134/134)
• npm run test:e2e with VS Code 1.90.2 (1/1)
• npm run package
• unzip -t mcpp-vscode-0.2.5.vsix
• verified the VSIX excludes test/** and dist/test/**
• git diff --check