chore(setup): build miner from DragonX/drg-xmrig and rename staging dir

setup.sh now clones/builds the miner from git.dragonx.is/DragonX/drg-xmrig
(was dragonx/xmrig-hac), and the prebuilt staging dir is renamed
prebuilt-binaries/xmrig-hac -> prebuilt-binaries/drg-xmrig across build.sh, the
legacy Windows build script, .gitignore and README.

Also aligns setup.sh's section-8 directory list with section 7 (it previously
used a bare "xmrig" token, creating a stray empty prebuilt-binaries/xmrig/),
and updates the xmrig_manager.cpp header comment.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -81,8 +81,8 @@ Download linux and windows binaries of latest releases and place in binary direc
 - prebuilt-binaries/dragonxd-win/
 - prebuilt-binaries/dragonxd-mac/
 
-**xmrig HAC fork** (https://git.dragonx.is/dragonx/xmrig-hac):
-- prebuilt-binaries/xmrig-hac/
+**DRG-XMRig fork** (https://git.dragonx.is/DragonX/drg-xmrig):
+- prebuilt-binaries/drg-xmrig/
 
 
 ## Build Steps