Add PyDesk V1 architecture blueprint and working scaffold

[ethgr0wth]

Motivation

• Provide a minimal, production-aware vertical slice for a Python-first desktop platform ("PyDesk") so developers can scaffold, run, and build a desktop app with a Python-native bridge.
• Keep the prototype isolated from existing code by creating a self-contained folder with docs, CLI, templates, and an example app.

Description

• Add design and roadmap docs at agent-runtime-fabric/python-desktop-platform/README.md and docs/ describing product definition, architecture options, V1 scope, security model, packaging, and roadmap.
• Implement a tiny platform core package under platform_core/src/pydesk with pyproject.toml, a pydesk CLI (new, dev, build, doctor), and a TOML config loader (pydesk.core.config) that includes a tomli fallback for Python <3.11.
• Add a blank-app template (desktop shell using pywebview, backend/bridge.py, desktop/main.py, and frontend/index.html) and an example_app vertical slice demonstrating frontend→Python bridge calls and a sample platform.toml.
• Include a basic unit test for the config loader at platform_core/tests/test_config.py and clear developer README/run instructions and build stub using pyinstaller.

Testing

• Ran unit tests with PYTHONPATH=src python -m pytest -q in platform_core after adding tomli fallback, and the test suite passed (1 passed).
• Compiled the new modules with python -m compileall platform_core/src example_app/backend example_app/desktop and compilation succeeded with no syntax errors.
• Notes: an initial pytest run failed due to tomllib availability on Python 3.10, a tomli fallback was added and the failure was resolved; packaging and runtime smoke (GUI) were not executed in this environment.

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Codex Task