fix: MD5→SHA-256, shellcheck warnings, and broken doc links

config_service.py: replace MD5 with SHA-256 for config change detection;
same semantics (equality comparison), no stored hashes affected.

Shell scripts — shellcheck warnings:
- diagnose_web_interface.sh: remove useless cat (SC2002)
- dev_plugin_setup.sh: restructure A&&B||C into if/then (SC2015)
- fix_assets_permissions.sh: remove unused REAL_HOME block (SC2034)
- install_web_service.sh: remove unused USER_HOME assignment (SC2034)
- diagnose_web_ui.sh: remove unused SUDO assignments (SC2034)
- diagnose_plugin_permissions.sh: remove unused BLUE color var (SC2034)
- first_time_install.sh: remove unused CLEAR var, PACKAGE_NAME
  assignment, and replace loop variable with _ (SC2034)

docs/PLUGIN_ARCHITECTURE_SPEC.md: fix 10 broken TOC anchor links to
include section numbers matching the actual headings (MD051).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/dev/dev_plugin_setup.sh

@@ -203,7 +203,9 @@ link_github_plugin() {
         log_info "Repository already exists at $target_dir"
         if [[ -d "$target_dir/.git" ]]; then
             log_info "Updating repository..."
-            (cd "$target_dir" && git pull --rebase || true)
+            if cd "$target_dir"; then
+                git pull --rebase || true
+            fi
         fi
     else
         # Clone the repository