feat: add storage module with S3 support and development tooling

Create shared storage module for AWS S3 operations with comprehensive
development infrastructure:

Core Features:
- S3 interface with two upload patterns (manager and direct)
- Presigned URL generation with 15-minute expiration
- Support for multipart uploads and direct PutObject
- Comprehensive test coverage (8 tests, 70.4% coverage)
- Generic implementation without project-specific dependencies

Development Tooling:
- .editorconfig for consistent editor settings
- .pre-commit-config.yaml with Go linters and formatters
- .golangci.yml for golangci-lint configuration
- commitlint.config.js for conventional commit validation
- cliff.toml for automated changelog generation (v0.0.1)
- renovate.json for automated dependency updates
- .gitlab-ci.yml for CI/CD pipeline

CI/CD Pipeline:
- Automated testing with race detection
- Coverage tracking and Codecov integration
- Vulnerability scanning with govulncheck
- Pre-commit validation gates
- Release automation

Module exports:
- New(bucket) - Upload manager pattern for large files
- NewS3(cfg, bucket) - Direct upload pattern
- Store(path, content, contentType) - Upload and get presigned URL

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -0,0 +1,55 @@
+# Storage Module
+
+Shared storage utilities for AWS S3.
+
+## Features
+
+- S3 object storage with presigned URL generation
+- Two upload strategies: managed uploads (for large files) and direct uploads
+- Configurable part size for multipart uploads
+- 15-minute presigned URL expiration
+
+## Usage
+
+### Using the Upload Manager (recommended for large files)
+
+```go
+import "gitlab.com/unboundsoftware/storage"
+
+// Create storage with automatic AWS config loading
+s3Storage, err := storage.New("my-bucket")
+if err != nil {
+    // handle error
+}
+
+// Upload a file and get a presigned URL
+url, err := s3Storage.Store("path/to/file.pdf", fileReader, "application/pdf")
+```
+
+### Using Direct Upload (for smaller files or custom config)
+
+```go
+import (
+    "github.com/aws/aws-sdk-go-v2/config"
+    "gitlab.com/unboundsoftware/storage"
+)
+
+// Load custom AWS config
+cfg, err := config.LoadDefaultConfig(context.Background())
+if err != nil {
+    // handle error
+}
+
+// Create storage with custom config
+s3Storage := storage.NewS3(cfg, "my-bucket")
+
+// Upload a file and get a presigned URL
+url, err := s3Storage.Store("path/to/file.pdf", fileReader, "application/pdf")
+```
+
+## Configuration
+
+The storage module uses AWS SDK v2 and loads configuration from:
+- Environment variables (`AWS_REGION`, `AWS_ACCESS_KEY_ID`, `AWS_SECRET_ACCESS_KEY`)
+- Shared configuration files (`~/.aws/config`, `~/.aws/credentials`)
+- IAM roles (when running on AWS infrastructure)