fix(FN-1619): convert remaining engine execSync calls to async execAsync

- Replace blocking execSync calls with async execAsync in executor.ts for user-configured commands
- Convert self-healing.ts worktree status checks to async to avoid blocking the event loop
- Update step-session-executor.ts to use async worktree operations
- Refactor worktree-pool.ts for fully async worktree creation, cleanup, and listing
- Convert pi.ts agent session handling to async execution
- Update all corresponding tests with async/await patterns
- Add changeset for @gsxdsm/fusion patch release

.changeset/convert-remaining-engine-exec-sync.md

@@ -0,0 +1,5 @@
+---
+"@gsxdsm/fusion": patch
+---
+
+Convert remaining execSync calls in engine to async execAsync to prevent event loop blocking.