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/logger/zap_logger.go

@@ -8,11 +8,7 @@ import (
 	"go.uber.org/zap/zapcore"
 )
 
-const (
-	// Context keys for extracting values from context
-	requestIDKey = "request_id"
-	userIDKey    = "user_id"
-)
+// Note: contextKey constants are defined in middleware.go to avoid duplication
 
 // zapLogger implements the Logger interface using zap.
 type zapLogger struct {
@@ -122,12 +118,12 @@ func convertFields(fields []logger.Field) []zap.Field {
 
 // RequestIDKey returns the context key for request ID.
 // This is exported so modules can use it to set request IDs in context.
-func RequestIDKey() string {
+func RequestIDKey() ContextKey {
 	return requestIDKey
 }
 
 // UserIDKey returns the context key for user ID.
 // This is exported so modules can use it to set user IDs in context.
-func UserIDKey() string {
+func UserIDKey() ContextKey {
 	return userIDKey
 }