Restores the package removed in 5ed483f0d7, but with a maintained fork
rather than abdanoned repo:
- Upstream phacility/arcanist is archived; use its maintained
continuation, the phorgeit fork, at current master
(0-unstable-2026-07-19).
- Regenerate the python3-in-PATH patch (`runWorkflow` visibility
changed) and drop `shellcomplete-strlen-null.patch` (now merged
upstream, it seems).
- Put in `pkgs/by-name` (unlike before), refresh `meta` for Phorge, set
`mainProgram`, treefmt.
Assisted-by: Claude Code (Claude Fable 5)
