fix: resolve all golangci-lint issues

- Add package comments to all packages (pkg/config, pkg/logger, internal/*, cmd/platform)

- Fix context key warnings by using custom ContextKey type
  - Export ContextKey type to avoid unexported-return warnings
  - Update all context value operations to use ContextKey instead of string
  - Update RequestIDKey() and UserIDKey() to return ContextKey

- Fix error checking issues (errcheck)
  - Properly handle os.Chdir errors in defer statements
  - Properly handle os.Setenv/os.Unsetenv errors in tests

- Fix security warnings (gosec)
  - Change directory permissions from 0755 to 0750 in tests
  - Change file permissions from 0644 to 0600 in tests

- Fix unused parameter warnings (revive)
  - Replace unused parameters with _ in:
    * RegisterLifecycleHooks lifecycle functions
    * Mock logger implementations
    * noOpLogger methods

- Fix type assertion issues (staticcheck)
  - Remove unnecessary type assertions in tests
  - Use simpler compile-time checks

- Fix exported type stuttering warning
  - Add nolint directive for ConfigProvider (standard interface pattern)

- Update golangci-lint configuration
  - Add version: 2 field (required for newer versions)
  - Remove unsupported linters (typecheck, gosimple)
  - Move formatters (gofmt, goimports) to separate formatters section
  - Simplify linter list to only well-supported linters

All linting issues resolved (0 issues reported by golangci-lint).
All tests pass and code compiles successfully.

internal/di/providers.go

@@ -67,13 +67,13 @@ func CoreModule() fx.Option {
 // RegisterLifecycleHooks registers lifecycle hooks for logging.
 func RegisterLifecycleHooks(lc fx.Lifecycle, l logger.Logger) {
 	lc.Append(fx.Hook{
-		OnStart: func(ctx context.Context) error {
+		OnStart: func(_ context.Context) error {
 			l.Info("Application starting",
 				logger.String("component", "bootstrap"),
 			)
 			return nil
 		},
-		OnStop: func(ctx context.Context) error {
+		OnStop: func(_ context.Context) error {
 			l.Info("Application shutting down",
 				logger.String("component", "bootstrap"),
 			)