feat(coding-agent): implemented downshift boomerang flow for context handoff

- Introduced `--downshift-boomerang` CLI flag and `downshift.boomerang` configuration setting.
- Integrated boomerang validation hand-back logic and context cutting within the agent session.
- Added system prompt templates and runtime checks to facilitate message handling for boomerang transitions.
- Included unit tests to verify context management and validation pass requirements during the boomerang process.

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

@@ -30,6 +30,7 @@ export interface Args {
 	downshift?: boolean;
 	noDownshift?: boolean;
 	downshiftInto?: string;
+	downshiftBoomerang?: boolean;
 	planYolo?: boolean;
 	planYoloInto?: string;
 	maxTime?: number;
@@ -240,6 +241,8 @@ 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

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