feat: removed boomerang feature and associated validation workflows

- Removed the `downshift.boomerang` feature, including CLI flags, configuration schema settings, and internal session state logic.
- Deleted the associated prompt definition file and all unit tests related to the boomerang validation flow.
- Cleaned up frontend forms and server request handling in the harbor-manager package to reflect the feature removal.
- Updated the downshift planning prompt instructions to maintain continuity in task creation.

packages/coding-agent/src/cli/args.ts

@@ -30,7 +30,6 @@ export interface Args {
 	downshift?: boolean;
 	noDownshift?: boolean;
 	downshiftInto?: string;
-	downshiftBoomerang?: boolean;
 	planYolo?: boolean;
 	planYoloInto?: string;
 	maxTime?: number;
@@ -241,8 +240,6 @@ export function parseArgs(inputArgs: string[], extensionFlags?: Map<string, { ty
 			result.downshift = true;
 		} else if (arg === "--no-downshift") {
 			result.noDownshift = true;
-		} else if (arg === "--downshift-boomerang") {
-			result.downshiftBoomerang = true;
 		} else if (arg === "--plan-yolo") {
 			result.planYolo = true;
 		} else if (arg === "--print" || arg === "-p") {

packages/coding-agent/src/cli/flag-tables.ts

@@ -283,7 +283,6 @@ export const VALUELESS_FLAGS: ReadonlySet<string> = new Set([
 	"--advisor",
 	"--downshift",
 	"--no-downshift",
-	"--downshift-boomerang",
 	"--plan-yolo",
 	"--print",
 	"--print-thoughts",