Add worktree port configuration docs to STACK.md

Documents STORYKIT_PORT env var for running multiple instances
in parallel worktrees without port conflicts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.story_kit/specs/tech/STACK.md

@@ -110,6 +110,15 @@ To support both Remote and Local models, the system implements a `ModelProvider`
     *   `vitest`: Unit/component testing.
     *   `playwright`: End-to-end testing.
 
+## Running the App (Worktrees & Ports)
+
+Multiple instances can run simultaneously in different worktrees. To avoid port conflicts:
+
+- **Backend:** Set `STORYKIT_PORT` to a unique port (default is 3001). Example: `STORYKIT_PORT=3002 cargo run`
+- **Frontend:** Run `pnpm dev` from `frontend/`. It auto-selects the next unused port. It reads `STORYKIT_PORT` to know which backend to talk to, so export it before running: `export STORYKIT_PORT=3002 && cd frontend && pnpm dev`
+
+When running in a worktree, use a port that won't conflict with the main instance (3001). Ports 3002+ are good choices.
+
 ## Safety & Sandbox
 1.  **Project Scope:** The application must strictly enforce that it does not read/write outside the `project_root` selected by the user.
 2.  **Human in the Loop:**