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/plugin-handlers/prometheus-agent-config-builder.ts

@@ -1,5 +1,6 @@
 import type { CategoryConfig } from "../config/schema";
 import { PROMETHEUS_PERMISSION, PROMETHEUS_SYSTEM_PROMPT } from "../agents/prometheus";
+import { resolvePromptAppend } from "../agents/builtin-agents/resolve-file-uri";
 import { AGENT_MODEL_REQUIREMENTS } from "../shared/model-requirements";
 import {
   fetchAvailableModels,
@@ -92,7 +93,7 @@ export async function buildPrometheusAgentConfig(params: {
   const { prompt_append, ...restOverride } = override;
   const merged = { ...base, ...restOverride };
   if (prompt_append && typeof merged.prompt === "string") {
-    merged.prompt = merged.prompt + "\n" + prompt_append;
+    merged.prompt = merged.prompt + "\n" + resolvePromptAppend(prompt_append);
   }
   return merged;
 }