The precon deck list was missing after the monolith refactor because
PRECON_DIR was hardcoded in services/precon_service.py with no CLI arg,
env var, or auto-resolve fallback — unlike forge_jar, forge_dir, etc.
Changes:
- Add precon_dir field to Config class (models/state.py)
- Add --precon-dir CLI arg and PRECON_DIR env var support
- Add _resolve_precon_dir() with project-root, parent-dir, and
forge_dir-relative detection; creates dir as fallback for downloads
- Refactor precon_service.py to use _get_precon_dir() helper that reads
CFG.precon_dir at runtime instead of a hardcoded module-level Path
- Update routes/precon.py to use _get_precon_dir() for install endpoints
- Wire _on_startup to resolve precon_dir for uvicorn-direct launches
- Remove unused PRECON_DIR/PRECON_INDEX imports from routes/lab.py
- Audit confirms all other CFG path fields already have fallbacks
