docs: ✍️ Scribe: Update project structure in README.md

[fderuiter]

💡 What: Updated the ASCII project structure tree in README.md to show the new packages/ monorepo layout (including core/, plugins-workflows/, and providers-airflow/) instead of the outdated imednet/ directory.
🎯 Why: The project recently migrated to a monorepo workspace structure. The README.md was still incorrectly pointing users to the imednet/ directory as the SDK package, which no longer exists at the root level.
🧠 Cognitive Impact: Prevents confusion and broken onboarding paths for new developers. By accurately mapping the project structure, developers can immediately understand where the core SDK lives vs. workflow plugins and Airflow providers, significantly reducing their "Time to Understand".
📖 Preview:

.
├── docs/       - Sphinx documentation
├── examples/   - Usage samples
├── packages/   - Workspace packages
│   ├── core/              - Main SDK package
│   ├── plugins-workflows/ - Workflow plugins package
│   └── providers-airflow/ - Airflow providers package
├── scripts/    - Helper scripts
└── tests/      - Unit and integration tests

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PR created automatically by Jules for task 935352066392667647 started by @fderuiter

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