Add --diff mode for update preview and hardcode org names configurable #15
Description
Overview
Two UX/portability improvements:
1. `--diff` mode for update previews
Currently `--dry-run` tells you what would change but not the actual content diff. Users must blindly trust regeneration output.
Fix: Add `--diff` that writes rendered output to temp file and runs `diff -u` against existing.
2. Hardcoded org names in template
The git config step hardcodes `open-runtime/` and `pieces-app/` org names. Not configurable via config.json.
Fix: Make org list configurable: `"git_orgs": ["open-runtime", "pieces-app"]` and render dynamically.
Acceptance Criteria
- `--diff` flag shows unified diff of old vs new content
- Git config org names driven by config, not hardcoded in template