refactor(devtools): modularize by package manager

[Sstark97]

Summary

Split devtools.sh (598 lines) into isolated modules per package manager, following Single Responsibility Principle. Each module is independently sourceable and testable. devtools.sh now acts as a lightweight orchestrator.

Changes

• New modules in src/modules/package-managers/:
  • maven.sh — Maven cache cleanup
  • gradle.sh — Gradle cache cleanup (outdated versions + complete)
  • node.sh — Node.js caches (npm, yarn, pnpm) + NVM
  • python.sh — Python caches (pip, poetry)
  • sdkman.sh — SDKMAN temp files
• devtools.sh reduced to orchestrator: sources all modules, exposes devtools_clean() as the single entry point
• Removed dead code: unused helper functions (get_npm_cache_info, get_yarn_cache_info, get_pnpm_cache_info, get_pip_cache_info, get_poetry_cache_info) and unused constant NODE_CACHE_SIZE_THRESHOLD_MB

What stays the same

• External API (devtools_clean()) unchanged — no impact on bin/devsweep or menu.sh
• Cleanup execution order preserved: Maven → Gradle → Node.js → NVM → SDKMAN → Python
• All three modes respected per module: normal, dry-run, analyze
• Double-source guards on every module

Test results

All 126 tests passing (115 unit + 11 e2e), including all existing devtools_test.sh and analyze_mode_test.sh tests without modifications.

Closes #9