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

@@ -0,0 +1,16 @@
+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