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/config/schema/agent-overrides.ts

@@ -12,6 +12,7 @@ export const AgentOverrideConfigSchema = z.object({
   temperature: z.number().min(0).max(2).optional(),
   top_p: z.number().min(0).max(1).optional(),
   prompt: z.string().optional(),
+  /** Text to append to agent prompt. Supports file:// URIs (file:///abs, file://./rel, file://~/home) */
   prompt_append: z.string().optional(),
   tools: z.record(z.string(), z.boolean()).optional(),
   disable: z.boolean().optional(),