feat(store): implement dynamic database path selection

- Prioritize local `.store/documents.db` if it exists for development.
- Fallback to standard OS application data directory using `env-paths`.
- Add tests using `memfs` to verify path selection logic.
- Update ARCHITECTURE.md and README.md with details.

Author: arabold

ARCHITECTURE.md

@@ -151,7 +151,14 @@ graph TD
 
 ### Document Storage Design
 
-The project uses SQLite for document storage, providing a lightweight and efficient database solution that requires no separate server setup. Documents are stored with URLs and sequential ordering to maintain source context:
+The project uses SQLite for document storage, providing a lightweight and efficient database solution that requires no separate server setup.
+
+**Database Location:** The application determines the database file (`documents.db`) location dynamically:
+
+1. It first checks for a `.store` directory in the current working directory (`process.cwd()`). If `.store/documents.db` exists, it uses this path. This prioritizes local development databases.
+2. If the local `.store/documents.db` does not exist, it defaults to a standard, OS-specific application data directory (e.g., `~/Library/Application Support/docs-mcp-server/` on macOS, `~/.local/share/docs-mcp-server/` on Linux) determined using the `env-paths` library. This ensures a stable, persistent location when running via `npx` or outside a local project context.
+
+Documents are stored with URLs and sequential ordering to maintain source context:
 
 ```mermaid
 graph LR