feat: implement comprehensive asset shipping for md-render command

Add automatic asset copying when rendering markdown to different output
directories with intelligent defaults and full user control.

Key Features:
- Environment variable support: MARKITECT_OUTPUT_DIR sets default output directory
- Smart defaults: auto-ship assets for directory output, disabled for file output
- CLI control flags: --ship-assets and --no-ship-assets for explicit control
- Timestamp-based copying: only copies when source newer than destination
- Path preservation: maintains relative directory structure in output
- Graceful error handling: missing assets logged as warnings, not failures

Technical Implementation:
- Enhanced asset discovery in markitect/assets/discovery.py with discover_assets_from_markdown()
- Added environment variable priority: CLI --output > MARKITECT_OUTPUT_DIR > input directory
- Comprehensive asset shipping logic with _ship_assets() function
- Directory vs file output detection for intelligent default behavior

Examples and Testing:
- Added image-assets example directory with 6 sample images and comprehensive README
- Created comprehensive TDD test suite with 10 tests covering all functionality
- Tests validate environment variables, CLI flags, asset discovery, shipping logic,
  timestamp handling, missing assets, path preservation, and default behaviors

Usage:
  markitect md-render file.md -o /output/dir/     # Auto-ships assets
  markitect md-render file.md --no-ship-assets   # Suppresses shipping
  MARKITECT_OUTPUT_DIR=/docs markitect md-render file.md  # Uses env var

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

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

examples/image-assets/README.txt

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+Image Asset Management Examples
+
+This directory contains examples demonstrating MarkiTect's image asset management
+capabilities:
+
+- project_documentation.md: Sample project documentation with embedded images
+  showing how MarkiTect handles image assets in markdown documents
+- images/: Directory containing sample images used in the documentation examples
+
+These examples showcase:
+- Image embedding in markdown documents
+- Asset deduplication and content-addressable storage
+- Relative path handling for images in MarkiTect projects
+- Best practices for organizing image assets in documentation
+
+--worsch, 25-10-29

examples/image-assets/project_documentation.md

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+# Project Documentation Example
+
+## Overview
+
+This document demonstrates MarkiTect's image asset management capabilities by embedding various types of images commonly used in technical documentation.
+
+## Architecture Diagram
+
+The following diagram shows the overall system architecture:
+
+![System Architecture](images/architecture_diagram.png)
+
+*Figure 1: High-level system architecture showing component interactions*
+
+## User Interface Screenshots
+
+### Dashboard View
+
+The main dashboard provides an overview of system status:
+
+![Dashboard Screenshot](images/dashboard_screenshot.png)
+
+*Figure 2: Main dashboard interface with key metrics and navigation*
+
+### Settings Panel
+
+Users can configure system behavior through the settings panel:
+
+![Settings Panel](images/settings_panel.png)
+
+*Figure 3: Configuration interface for system preferences*
+
+## Logo and Branding
+
+### Company Logo
+
+![Company Logo](images/company_logo.png)
+
+### Project Icon
+
+The project uses this icon throughout the interface:
+
+![Project Icon](images/project_icon.png)
+
+## Asset Management Features
+
+MarkiTect provides several key features for managing image assets:
+
+1. **Content-Addressable Storage**: Images are stored using SHA-256 hashes to prevent duplication
+2. **Automatic Deduplication**: Identical images are only stored once, regardless of filename
+3. **Relative Path Resolution**: Images can be referenced using relative paths from the markdown file
+4. **Asset Tracking**: All referenced assets are tracked and validated during document processing
+
+## Performance Metrics
+
+The following chart shows system performance over time:
+
+![Performance Chart](images/performance_chart.png)
+
+*Figure 4: System performance metrics showing response time and throughput*
+
+## Conclusion
+
+This example demonstrates how MarkiTect seamlessly handles multiple image assets within a single document, providing:
+
+- Efficient storage through deduplication
+- Reliable asset resolution
+- Clean integration with markdown syntax
+- Support for various image formats (PNG, JPG, SVG, etc.)
+
+All images in this document will be processed through MarkiTect's asset management system when the document is rendered or packaged.