chore(scripts): delete unreferenced helper scripts

None of these is referenced by an installer, systemd unit, CI workflow,
test, the web UI or src/:

- utils/cleanup_venv.sh removes venv_web_v2, which nothing creates
- utils/clear_python_cache.sh hardcodes ~/LEDMatrix and a .webassets-cache
  nothing uses
- install/migrate_config.sh only copies the template, which the installer
  and ConfigManager already do
- install/debug_install.sh, debug/debug_web_manual.py
- diagnose_web_ui.sh and verify_web_ui.sh overlap diagnose_web_interface.sh,
  which the docs point to
- fix_internet_connectivity.sh is iptables-only (stale on nftables)
- diagnose_plugin_permissions.sh, dev/validate_python.py
- download_nba_logos.py + README_NBA_LOGOS.md: logo_downloader fetches
  logos on demand
- setup_plugin_repos.py linked into the production plugin-repos/ dir; the
  dev workflow is scripts/dev/dev_plugin_setup.sh, and
  MULTI_ROOT_WORKSPACE_SETUP.md now uses it

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/install/README.md

@@ -19,9 +19,6 @@ This directory contains scripts for installing and configuring the LEDMatrix sys
   (the user who runs the script, i.e. the one you installed LEDMatrix as;
   there is no `ledmatrix` system user) the passwordless `nmcli` and related
   WiFi permissions the web interface needs
-- **`migrate_config.sh`** - Migrates configuration files to new formats (if needed)
-- **`debug_install.sh`** - Diagnostic helper used when an install
-  fails; collects environment info and recent logs
 
 ## Usage