feat: renamed subagent handoff flow to use yield instead of submit_result

- Renamed subagent completion flow from `submit_result` to `yield` across SDK tools, prompts, and docs.
- Updated executor/task handling to require and parse `yield` calls, replacing legacy submit-result extraction and state flags.
- Added `subagent-yield-reminder` and updated system prompts to require `yield` with `result.data` or `result.error`.
- Renamed hidden-tool and registration plumbing to `yield`, including discovery helpers and renderer/test surface.

packages/coding-agent/test/discovery/agent-fields.test.ts

@@ -64,6 +64,6 @@ describe("parseAgentFields", () => {
 			tools: ["Read", "Grep"],
 		});
 
-		expect(fields?.tools).toEqual(["read", "grep", "submit_result"]);
+		expect(fields?.tools).toEqual(["read", "grep", "yield"]);
 	});
 });