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>

TODO.md

@@ -29,6 +29,25 @@ This section is for tasks currently being discussed with or worked on by the cod
 
 ## Completed Tasks
 
+**Asset Shipping for md-render - COMPLETED ✅**:
+- ✅ Implemented automatic asset copying when rendering markdown to different output directories
+- ✅ Added asset discovery functionality parsing markdown for image/link references
+- ✅ Implemented timestamp-based asset copying (only copy if source newer than destination)
+- ✅ Added `--ship-assets` and `--no-ship-assets` CLI flags for explicit control
+- ✅ Added `MARKITECT_OUTPUT_DIR` environment variable support for default output directory
+- ✅ Smart defaults: assets ship automatically when output is directory, disabled for specific files
+- ✅ Preserved relative path structure in output directory maintaining markdown link compatibility
+- ✅ Graceful handling of missing assets with warning messages
+- ✅ Full backward compatibility with existing md-render workflows
+- ✅ Comprehensive TDD test suite covering all functionality and edge cases
+
+**Feature Capabilities**:
+- Environment variable priority: CLI `--output` > `MARKITECT_OUTPUT_DIR` > input file directory
+- Automatic asset discovery from standard markdown syntax: `![alt](path)` and `[text](path)`
+- Timestamp-based incremental copying prevents unnecessary file operations
+- Directory structure preservation maintains working relative links in output HTML
+- Support for images, documents, and other asset types referenced in markdown
+
 **CHANGELOG.md Enhancement - COMPLETED ✅**:
 - ✅ Added missing version entries for 0.1.0, 0.2.0, and 0.3.0
 - ✅ Added standard Keep a Changelog header with proper format