Refactor setup and tool helpers for improved modularity and clarity

- Moved setup logic into dedicated files: agent.ts, config.ts, index.ts, workspace.ts.
- Introduced tool-helpers.ts for shared functions across tools, reducing boilerplate.
- Updated tools (status, task-comment, task-create, task-update, work-finish, work-start) to utilize new helper functions for workspace resolution and provider creation.
- Enhanced error handling and context detection in tools.
- Improved project resolution logic to streamline tool execution.
- Added new functionality for agent creation and configuration management in setup.

lib/model-selector.ts

@@ -4,7 +4,7 @@
  * Returns tier names (junior, medior, senior, qa) instead of model aliases.
  */
 
-export type ModelRecommendation = {
+export type TierRecommendation = {
   tier: string;
   reason: string;
 };
@@ -47,11 +47,11 @@ const COMPLEX_KEYWORDS = [
  * - senior: deep/architectural (system-wide refactoring, novel design)
  * - qa: all QA tasks (code inspection, validation, test runs)
  */
-export function selectModel(
+export function selectTier(
   issueTitle: string,
   issueDescription: string,
   role: "dev" | "qa",
-): ModelRecommendation {
+): TierRecommendation {
   if (role === "qa") {
     return {
       tier: "qa",