Add fixed install mode for production deployments

[jensens]

Summary

Adds support for non-editable (fixed) installations to enable production and Docker deployments, addressing issue #54.

Changes

• New fixed install mode: Installs packages without -e prefix for production use
• New editable install mode: Installs packages with -e prefix (replaces direct as default)
• Deprecate direct mode: Kept as backward-compatible alias for editable with deprecation warning
• Keep skip mode: Unchanged behavior

Implementation Details

• Modified config.py to validate new modes and handle deprecation
• Modified processing.py to conditionally add -e prefix based on install mode
• Default install mode changed from direct to editable (same behavior, clearer naming)
• Deprecation warnings logged when direct mode is used (global or per-package)

Documentation

• Updated README.md with new install mode options
• Updated CLAUDE.md with examples and explanations
• Added CHANGES.md entry for version 4.1.2

Test Coverage

• Added 4 new test configuration files
• Added 4 new configuration tests for new modes and deprecation
• Added 2 new processing tests for fixed and mixed modes
• Updated 1 existing test for new default behavior
• All 172 tests passing ✓

Test-Driven Development

This PR was implemented using TDD methodology:

1. ✅ Red phase: Wrote failing tests first
2. ✅ Green phase: Implemented code to pass tests
3. ✅ Verification: All tests pass

Backward Compatibility

• Existing configurations using direct mode continue to work
• Deprecation warning logged to guide users to migrate to editable
• No breaking changes

Fixes #54