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]>

wiki/Home.md

@@ -151,15 +151,15 @@ These tools combine AI prompts with code for validation:
 
 ## Creating Your Own Tools
 
-### Using the TUI
+### Using the GUI
 
 The easiest way to create tools:
 
 ```bash
-cmdforge ui
+cmdforge
 ```
 
-Navigate with arrow keys, create tools visually, edit prompts, test with mock provider.
+Opens the graphical interface where you can create tools visually, edit prompts, manage providers, and browse the registry.
 
 ### Tool Anatomy (YAML)
 
@@ -245,10 +245,10 @@ cat important.txt | summarize --provider claude-opus
 
 ### Change Tool Default
 
-Edit the tool's config or use the TUI:
+Edit the tool's config or use the GUI:
 
 ```bash
-cmdforge ui
+cmdforge
 # Select tool → Edit → Change provider in step
 ```