Add Custom Slash Commands support

[PureWeen]

Summary

SDK v0.2.1 added CommandDefinition — register custom slash commands per session that trigger PolyPilot-specific behavior.

SDK API

• SessionConfig.Commands → CommandDefinition[] (v0.2.1)
• CommandDefinition — Name, Description, Handler
• Events: CommandQueuedEvent, CommandExecuteEvent, CommandCompletedEvent, CommandsChangedEvent
• RPC: session.Rpc.Commands.HandlePendingCommandAsync(id, response)

Proposed Implementation

1. Register PolyPilot-specific commands at session creation:
   • /deploy — trigger build & deploy workflow
   • /test — run project test suite
   • /review — run code review agent
   • /screenshot — capture app screenshot via MauiDevFlow
2. Command palette — show available commands with descriptions when user types /
3. Custom command editor — let users define their own commands per session/group
4. Command activity log — show when commands fire via CommandExecuteEvent

Priority: 🟡 MEDIUM