refactor: consolidate shared packages and eliminate code duplication

- Update 26+ files to import secureFs from @automaker/platform
- Create shared type files: github.ts, worktree.ts, claude.ts in libs/types
- Create exec-utils.ts for cross-platform shell execution
- Delete redundant wrapper files: secure-fs.ts, stream-processor.ts, enhancement-prompts.ts
- Update GitHub routes to use createLogError pattern
- Add isENOENT helper to routes/common.ts
- Fix test imports to use @automaker/prompts and @automaker/platform

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

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

apps/server/tests/unit/lib/enhancement-prompts.test.ts

@@ -15,7 +15,7 @@ import {
   SIMPLIFY_EXAMPLES,
   ACCEPTANCE_EXAMPLES,
   type EnhancementMode,
-} from '@/lib/enhancement-prompts.js';
+} from '@automaker/prompts';
 
 describe('enhancement-prompts.ts', () => {
   describe('System Prompt Constants', () => {