docs: add project documentation

PROJECT.md

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+# Project: harbor-ctl
+
+## Purpose
+
+Command-line interface for managing Harbor container registry. Provides quick access to common operations without requiring the web UI or manual API calls.
+
+## Scope
+
+### In Scope
+
+- Project, repository, and artifact listing
+- Vulnerability scanning and reporting
+- SBOM retrieval and display
+- Tag management
+- Artifact deletion
+- Project configuration (auto-scan, auto-sbom)
+
+### Out of Scope
+
+- User/group management (admin-only, rare)
+- System configuration (one-time setup)
+- Replication rule creation (complex, rare)
+- Garbage collection triggers (scheduled)
+
+## Success Criteria
+
+- Modular package structure for maintainability
+- No external dependencies (stdlib only)
+- All common daily operations accessible via CLI
+- Clear, scannable output for terminal use
+- Exit codes suitable for scripting
+- Credentials managed securely (no hardcoding)
+
+## Constraints
+
+- Python 3.10+ stdlib only
+- Harbor API v2.0 compatibility
+- Must work in minimal environments (no pip install required)
+- Legacy single-file script maintained for compatibility
+
+## Assumptions
+
+- Harbor instance is accessible over HTTPS
+- User has appropriate permissions for requested operations
+- Credentials file follows established format when present