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.ts

@@ -171,6 +171,7 @@ export async function createBuiltinAgents(
     availableAgents,
     availableSkills,
     mergedCategories,
+    directory,
     userCategories: categories,
   })
   if (atlasConfig) {