Convert TUI to PySide6 desktop GUI

Major UI overhaul replacing the urwid-based Terminal UI with a modern
PySide6 desktop application.

New GUI features:
- Sidebar navigation (My Tools, Registry, Providers)
- Tool Builder with visual form for creating/editing tools
- Registry browser with search and one-click install
- Provider management page
- Connect dialog for account pairing
- Publish dialog for sharing tools

- Keyboard shortcuts (Ctrl+N, Ctrl+S, Ctrl+R, Ctrl+1/2/3, Escape, Ctrl+Q)
- Window geometry persistence (remembers size/position)
- Modern clean stylesheet

Removed:
- ui.py, ui_snack.py, ui_registry.py, ui_urwid.py
- ui_urwid/ directory (urwid TUI implementation)

Updated:
- pyproject.toml: PySide6 now required, removed urwid
- CLI entry points to launch GUI
- All documentation (README, CLAUDE.md, AGENTS.md, wiki)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

src/cmdforge/gui/__init__.py

@@ -0,0 +1,18 @@
+"""PySide6 GUI for CmdForge."""
+
+import sys
+
+
+def run_gui():
+    """Launch the CmdForge GUI application."""
+    from PySide6.QtWidgets import QApplication
+    from .main_window import MainWindow
+
+    app = QApplication(sys.argv)
+    app.setApplicationName("CmdForge")
+    app.setOrganizationName("CmdForge")
+
+    window = MainWindow()
+    window.show()
+
+    return app.exec()