docs: update documentation for debian-configuration repo rename

- Root README.md: new, describes overall repo layout (proxmox/, pihole/,
  scripts/) and secret-scanning workflow
- Root CLAUDE.md: new, cross-section guardrails and instructions for
  adding new machine types; references proxmox/CLAUDE.md and pihole/
  section-specific rules
- pihole/README.md: new, covers scripts usage, notable DHCP/PXE config,
  EFI VM requirements (VirtIO RNG, Secure Boot), and secrets handling
- proxmox/README.md: add repo-context note pointing back to root README

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XRzqNDrbnYR22ZgZj1Bg3s

proxmox/README.md

@@ -1,3 +1,5 @@
+> Part of the [debian-configuration](../README.md) repo — see the root README for repo-wide layout and secret-scanning setup.
+
 # Proxmox Configuration
 
 Base configuration and hardening toolset for Proxmox VE hosts, plus planning