Add skill to convert GSD codebase mapping (.planning/codebase/*.md)
to Autocoder app_spec.txt format.
This enables onboarding existing projects to Autocoder without
manually writing the XML spec.
Workflow:
1. Run /gsd:map-codebase on existing project
2. Run /gsd-to-autocoder-spec to generate app_spec.txt
3. Start Autocoder normally
Closes#55
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>