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/modes/components/settings-defs.ts

@@ -92,6 +92,13 @@ const CONDITIONS: Record<string, () => boolean> = {
 			return false;
 		}
 	},
+	downshiftEnabled: () => {
+		try {
+			return Settings.instance.get("downshift.enabled") === true;
+		} catch {
+			return false;
+		}
+	},
 	hindsightActive: () => {
 		try {
 			return Settings.instance.get("memory.backend") === "hindsight";