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/src/discovery/helpers.ts

@@ -221,9 +221,9 @@ export function parseAgentFields(frontmatter: Record<string, unknown>): ParsedAg
 
 	let tools = parseArrayOrCSV(frontmatter.tools)?.map(tool => tool.toLowerCase());
 
-	// Subagents with explicit tool lists always need submit_result
-	if (tools && !tools.includes("submit_result")) {
-		tools = [...tools, "submit_result"];
+	// Subagents with explicit tool lists always need yield
+	if (tools && !tools.includes("yield")) {
+		tools = [...tools, "yield"];
 	}
 
 	// Parse spawns field (array, "*", or CSV)