feat: refactor plugin architecture, enhance linting, CI & tooling

- Major refactor of core Fish functions for modularity, caching, and error handling
- Improved `.editorconfig` and Makefile for stricter formatting and linting standards
- Expanded linting support: added EditorConfig checks, auto-install for missing tools, and Makefile targets
- Enhanced CI workflow with clearer permissions and job steps in GitHub Actions
- Updated documentation in `README.md` and `CLAUDE.md` to reflect new features, advanced developer tools, and contribution guidelines
- Improved Node.js version manager detection, switching, and installation logic
- Added/updated utility functions for configuration, silent mode, notifications, and version extraction
- Various bug fixes, code quality improvements, and expanded test coverage

tests/unit/test_cache_helpers.fish

@@ -0,0 +1,99 @@
+#!/usr/bin/env fish
+# Unit tests for nvm_cache helper functions
+
+source tests/test_runner.fish
+
+function test_nvm_cache_get_set_delete
+    echo "Testing _nvm_cache_set, _nvm_cache_get, and _nvm_cache_delete..."
+
+    set -l key test_key
+    set -l value test_value
+
+    # Set cache value
+    _nvm_cache_set $key $value
+    set -l result (_nvm_cache_get $key 300)
+    assert_equals "$result" "$value" "Cache set and get returns correct value"
+
+    # Delete cache value
+    _nvm_cache_delete $key
+    set -l result (_nvm_cache_get $key 300)
+    set -l status_code $status
+    test $status_code -ne 0
+    and echo "✅ Cache delete works"
+    or echo "❌ Cache delete failed"
+
+    return 0
+end
+
+function test_nvm_cache_clear_and_stats
+    echo "Testing _nvm_cache_clear and _nvm_cache_stats..."
+
+    # Set multiple cache values
+    _nvm_cache_set key1 value1
+    _nvm_cache_set key2 value2
+
+    # Stats should show at least 2 files
+    set -l stats (_nvm_cache_stats)
+    assert_contains "$stats" "Cache files:" "Cache stats reports file count"
+
+    # Clear cache
+    _nvm_cache_clear
+    set -l stats_after (_nvm_cache_stats)
+    assert_contains "$stats_after" "Cache files: 0" "Cache clear removes all files"
+
+    return 0
+end
+
+function test_nvm_cache_ttl
+    echo "Testing _nvm_cache_get TTL expiration..."
+
+    set -l key ttl_key
+    set -l value ttl_value
+
+    _nvm_cache_set $key $value
+
+    # Should exist immediately
+    set -l result (_nvm_cache_get $key 10)
+    assert_equals "$result" "$value" "Cache value exists within TTL"
+
+    # Simulate expired cache by setting TTL to 0
+    set -l result (_nvm_cache_get $key 0)
+    set -l status_code $status
+    test $status_code -ne 0
+    and echo "✅ Cache TTL expiration works"
+    or echo "❌ Cache TTL expiration failed"
+
+    _nvm_cache_delete $key
+
+    return 0
+end
+
+function test_nvm_cache_dir
+    echo "Testing _nvm_cache_dir returns a valid directory..."
+
+    set -l dir (_nvm_cache_dir)
+    test -n "$dir"
+    and echo "✅ _nvm_cache_dir returns: $dir"
+    or echo "❌ _nvm_cache_dir did not return a directory"
+
+    return 0
+end
+
+function main
+    set -l failed 0
+
+    test_nvm_cache_get_set_delete; or set failed (math "$failed + 1")
+    test_nvm_cache_clear_and_stats; or set failed (math "$failed + 1")
+    test_nvm_cache_ttl; or set failed (math "$failed + 1")
+    test_nvm_cache_dir; or set failed (math "$failed + 1")
+
+    if test $failed -eq 0
+        echo "🎉 All nvm_cache helper tests passed!"
+        return 0
+    else
+        echo "💥 $failed nvm_cache helper test(s) failed"
+        return 1
+    end
+end
+
+main