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>

src/config_service.py

@@ -114,9 +114,9 @@ class ConfigService:
             self._start_file_watching()
     
     def _calculate_checksum(self, config: Dict[str, Any]) -> str:
-        """Calculate MD5 checksum of configuration."""
+        """Calculate checksum of configuration for change detection."""
         config_str = json.dumps(config, sort_keys=True)
-        return hashlib.md5(config_str.encode()).hexdigest()
+        return hashlib.sha256(config_str.encode()).hexdigest()
     
     def _load_config(self) -> bool:
         """