Refactor repository management for parallel workflow

Implemented new architecture where common component is built once and
merged with release-specific components, eliminating duplicate work
when running parallel workers.

Key changes:
- Workers in isolated mode (-R flag) now skip building common component
- process_release() returns early in isolated mode without publishing
- Added merge_repos() function to combine common + release-specific components
- merge_repos() imports from isolated DBs and publishes complete repos
- Updated help documentation with new 3-step workflow

Benefits:
- No duplicate work: common packages added once, not per release
- Faster parallel execution: each worker does less work
- Clean separation: common built separately, merged at the end
- Proper repository metadata: final merge uses aptly correctly

Signed-off-by: Igor Pecovnik <igor@armbian.com>

Author: igorpecovnik