feat(council-archive): add shared extraction utility and background task type extensions

- Create council-response-extractor.ts with ported extractCouncilResponse()
- Add writeOutputToFile, outputFilePath, _isCompleting to BackgroundTask
- Add writeOutputToFile to LaunchInput
- 8/8 extraction tests passing

src/features/background-agent/types.ts

@@ -63,6 +63,12 @@ export interface BackgroundTask {
   isUnstableAgent?: boolean
   /** Category used for this task (e.g., 'quick', 'visual-engineering') */
   category?: string
+  /** Flag to write raw output to file on completion */
+  writeOutputToFile?: boolean
+  /** Path to the output file after successful write */
+  outputFilePath?: string
+  /** Race condition guard: prevents concurrent re-entry during async completion */
+  _isCompleting?: boolean
 
   /** Last message count for stability detection */
   lastMsgCount?: number
@@ -89,6 +95,8 @@ export interface LaunchInput {
   skillContent?: string
   category?: string
   sessionPermission?: SessionPermissionRule[]
+  /** Flag to write raw output to file on completion */
+  writeOutputToFile?: boolean
 }
 
 export interface ResumeInput {