refactor: rename startinglineproductions to plan.startinglineproductions.com

- Rename mdbook source directory
- Rename Grav environment directory in container
- Update setup.php hostname mapping
- Update configure-sites.sh
- Update etc-hosts-entries.txt
- Update README.md site inventory

Consistent naming convention: all plan sites now use plan.{domain}

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Assisted-by: GLM-5 via Crush <crush@charm.land>

etc-hosts-entries.txt

@@ -26,4 +26,4 @@
 127.0.0.1 plan.thepeernet.com
 127.0.0.1 plan.turnsys.com
 127.0.0.1 plan.yourdreamnamehere.com
-127.0.0.1 startinglineproductions.com-bizopprodplan
+127.0.0.1 plan.startinglineproductions.com