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/sisyphus-junior/default.ts

@@ -7,6 +7,8 @@
  * - Extended reasoning context for complex tasks
  */
 
+import { resolvePromptAppend } from "../builtin-agents/resolve-file-uri"
+
 export function buildDefaultSisyphusJuniorPrompt(
   useTaskSystem: boolean,
   promptAppend?: string
@@ -40,7 +42,7 @@ Task NOT complete without:
 </Style>`
 
   if (!promptAppend) return prompt
-  return prompt + "\n\n" + promptAppend
+  return prompt + "\n\n" + resolvePromptAppend(promptAppend)
 }
 
 function buildConstraintsSection(useTaskSystem: boolean): string {

src/agents/sisyphus-junior/gpt.ts

@@ -16,6 +16,8 @@
  * - Explicit decision criteria needed (model won't infer)
  */
 
+import { resolvePromptAppend } from "../builtin-agents/resolve-file-uri"
+
 export function buildGptSisyphusJuniorPrompt(
   useTaskSystem: boolean,
   promptAppend?: string
@@ -85,7 +87,7 @@ Task NOT complete without evidence:
 </style_spec>`
 
   if (!promptAppend) return prompt
-  return prompt + "\n\n" + promptAppend
+  return prompt + "\n\n" + resolvePromptAppend(promptAppend)
 }
 
 function buildGptBlockedActionsSection(useTaskSystem: boolean): string {