feat: add verbose installer output

[terminalchai]

Closes #39

Summary

• add a --verbose CLI flag
• pass verbose mode through to runInstaller
• switch installer commands to stdio: 'inherit' when verbose mode is enabled
• add focused installer tests for verbose vs default stdio behavior

Validation

• npx jest tests/installer.test.js -t "uses inherited stdio when verbose mode is enabled|uses piped stdio by default" ?

Notes

• full repo validation is still not green on the current baseline outside this change:
  • npx jest --coverage still fails because of existing failures in tests/scriptCoverage.test.js, tests/installer.test.js, and existing coverage-threshold misses
  • npx eslint src/ index.js tests/ still fails because ESLint cannot find a configuration file for that command path in the checked repo state