feat(agents): add file:// URI support in prompt_append configuration

Port devxoul's PR #821 feature to current codebase structure.
Supports absolute, relative, ~/home paths with percent-encoding.
Gracefully handles malformed URIs and missing files with warnings.

Co-authored-by: devxoul <devxoul@gmail.com>

src/agents/builtin-agents/hephaestus-agent.ts

@@ -85,7 +85,7 @@ export function maybeCreateHephaestusConfig(input: {
   }
 
   if (hephaestusOverride) {
-    hephaestusConfig = mergeAgentConfig(hephaestusConfig, hephaestusOverride)
+    hephaestusConfig = mergeAgentConfig(hephaestusConfig, hephaestusOverride, directory)
   }
   return hephaestusConfig
 }