refactor: lean down src/index.ts after extraction PRs (#418)

[coder-Yash886]

Closes #418

Summary

This PR performs the final cleanup pass on src/index.ts following earlier extraction PRs, reducing src/index.ts from 974 lines down to a clean, readable 192-line orchestration layer.

Key Changes

1. Single-Project Scan Coordination:

   • Extracted handleSingleFolderScan and scanProject into src/scan/single-scan.ts (paralleling src/scan/multi-folder-scan.ts).
   • Encapsulates advisory source setup, package loading, vulnerability scanning, override hygiene checking, fix runner, PR creation, ratchet/baseline gating, maintenance/license checks, and output reporting.

2. Command Handlers & CLI Extraction:

   • src/cli/advisories-sync.ts: Extracted runAdvisoriesSync() for advisory syncing logic.
   • src/cli/advisories-init.ts: Added runAdvisoriesInit() helper for empty DB creation and console output.
   • src/cli/config-command.ts: Added handleConfigCommand() for subcommand validation and dispatch.
   • src/cli/config.ts: Added setupCaCert() to handle CA cert loading, validation, and cert-aware fetch setup.

3. Entry Point (src/index.ts):

   • main() is now purely focused on top-level dispatch, argument validation, and error handling with no inline user-facing strings or misplaced scan pipeline logic.

4. Cross-Platform Compatibility:

   • src/utils/file.ts: Updated directory depth calculations in chooseBestLockfile and findFiles to split on /[\\/]/ for consistent behavior across POSIX and Windows.

Verification

• npm run build passes with zero errors and clean bundle verification (dist/index.js).
• node --experimental-vm-modules ./node_modules/jest/bin/jest.js tests/cli-integration.test.ts (42/42 tests pass).
• node --experimental-vm-modules ./node_modules/jest/bin/jest.js tests/cli (88/88 tests pass).
• node --experimental-vm-modules ./node_modules/jest/bin/jest.js tests/e2e/commands-and-exit-codes.test.ts (21/21 tests pass).
• Pure refactoring with zero breaking changes or behavioral changes.

[coder-Yash886]

@sonukapoor Please review the PR when you have free time