Refactor main.ts to use SDK createAgentSession

- Add continueSession, restoreFromSession, additionalHookPaths,
  additionalCustomToolPaths, scopedModels to CreateAgentSessionOptions
- Return CreateAgentSessionResult with session, customToolsResult,
  and modelFallbackMessage
- main.ts now focuses on CLI arg parsing and mode routing
- Reduced main.ts from ~490 to ~340 lines

packages/coding-agent/src/index.ts

@@ -89,6 +89,7 @@ export {
 	type BuildSystemPromptOptions,
 	buildSystemPrompt,
 	type CreateAgentSessionOptions,
+	type CreateAgentSessionResult,
 	// Factory
 	createAgentSession,
 	// Helpers