fix: respect XDG_CACHE_HOME for model cache directory

MODEL_CACHE_DIR was hardcoded to ~/.cache/qmd/models/, ignoring the
XDG_CACHE_HOME environment variable. This was inconsistent with the rest
of the codebase (store.ts, cli/qmd.ts) which already respects XDG paths.

Fixes #425

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

src/llm.ts

@@ -209,7 +209,9 @@ export const DEFAULT_RERANK_MODEL_URI = DEFAULT_RERANK_MODEL;
 export const DEFAULT_GENERATE_MODEL_URI = DEFAULT_GENERATE_MODEL;
 
 // Local model cache directory
-const MODEL_CACHE_DIR = join(homedir(), ".cache", "qmd", "models");
+const MODEL_CACHE_DIR = process.env.XDG_CACHE_HOME
+  ? join(process.env.XDG_CACHE_HOME, "qmd", "models")
+  : join(homedir(), ".cache", "qmd", "models");
 export const DEFAULT_MODEL_CACHE_DIR = MODEL_CACHE_DIR;
 
 export type PullResult = {