docs: simplify README and remove FreeBSD support

- Strip README to essentials with wiki references only
- Remove non-existent API docs and troubleshooting links
- Focus on quick start and actual integrations (merkwerk)
- Remove FreeBSD support from all installation scripts
- Clean up platform detection logic in scripts
- Maintain OpenBSD and Linux support only

Reduces maintenance burden and aligns with actual project scope.

scripts/setup-directories.sh

@@ -4,7 +4,7 @@
 set -e
 
 # Detect operating system for config directory
-if [ "$(uname)" = "OpenBSD" ] || [ "$(uname)" = "FreeBSD" ]; then
+if [ "$(uname)" = "OpenBSD" ]; then
     CONFIG_DIR="/usr/local/etc/furt"
     USER="_furt"
     GROUP="_furt"