feat(glyph): populate Open WebUI system prompt from llm-profile flake input

Set DEFAULT_SYSTEM_PROMPT by reading the llm-profile flake input
(github:stackptr/llm-profile) at build time. Updating the prompt
only requires bumping the flake input and rebuilding, with no manual
secret editing or database changes.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: stackptr

hosts/glyph/services/open-webui.nix

@@ -1,5 +1,6 @@
 {
   config,
+  inputs,
   pkgs,
   ...
 }: {
@@ -40,6 +41,9 @@
       ENABLE_API_KEYS = "True";
       USER_PERMISSIONS_FEATURES_API_KEYS = "True";
       DATABASE_URL = "postgresql:///open-webui?host=/run/postgresql";
+
+      # System prompt sourced from llm-profile flake input (github:stackptr/llm-profile)
+      DEFAULT_SYSTEM_PROMPT = builtins.readFile "${inputs.llm-profile}/README.md";
     };
   };
 }