refactor: consolidate git utilities and model constants

COMPLETED MIGRATIONS:
- Migrate git utilities from routes/common.ts (383 lines → 39 lines)
  - Replace duplicated code with imports from @automaker/git-utils
  - Keep only route-specific utilities (getErrorMessage, createLogError)
  - All git operations now use shared package consistently

- Remove duplicate model constants in UI
  - Update model-config.ts to import from @automaker/types
  - Update agent-context-parser.ts to use DEFAULT_MODELS.claude
  - Removed 40+ lines of duplicated code

DEFERRED (Server-Specific):
- enhancement-prompts.ts (456 lines) - Server-only, no UI usage
- app-spec-format.ts (318 lines) - Server-only, no UI usage

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/ui/src/lib/agent-context-parser.ts

@@ -3,6 +3,8 @@
  * Extracts useful information from agent context files for display in kanban cards
  */
 
+import { DEFAULT_MODELS } from "@automaker/types";
+
 export interface AgentTaskInfo {
   // Task list extracted from TodoWrite tool calls
   todos: {
@@ -27,7 +29,7 @@ export interface AgentTaskInfo {
 /**
  * Default model used by the feature executor
  */
-export const DEFAULT_MODEL = "claude-opus-4-5-20251101";
+export const DEFAULT_MODEL = DEFAULT_MODELS.claude;
 
 /**
  * Formats a model name for display