feat(coding-agent): added support for task-agent field and model resolution

- Added schema and type updates for task-agent fields and model resolver settings.
- Extended discovery helper logic to carry resolved task-agent metadata through execution setup.
- Updated task/agent registration and execution paths to use the new capability/field data.
- Expanded test coverage for agent-field parsing, model resolution, and executor prewalk behavior.

packages/coding-agent/src/task/index.ts

@@ -1265,6 +1265,9 @@ export class TaskTool implements AgentTool<TaskToolSchemaInstance, TaskToolDetai
 					systemPrompt: `${planModeSubagentPrompt}\n\n${agent.systemPrompt}`,
 					tools: planModeTools,
 					spawns: undefined,
+					// Read-only exploration: never arm prewalk (its plan/implement
+					// nudges assume edit tools the plan-mode toolset doesn't have).
+					prewalk: undefined,
 				}
 			: agent;