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/general-agents.ts

@@ -84,7 +84,7 @@ export function collectPendingBuiltinAgents(input: {
       config = applyEnvironmentContext(config, directory)
     }
 
-    config = applyOverrides(config, override, mergedCategories)
+    config = applyOverrides(config, override, mergedCategories, directory)
 
     // Store for later - will be added after sisyphus and hephaestus
     pendingAgentConfigs.set(name, config)