Implement slash command system for Ink TUI

[randomm]

Phase 3 of Ink TUI Rewrite

Objective

Port existing slash command system to Ink architecture.

Commands to Implement

Command	Description
/help	Show help
/clear	Clear screen
/agent [name]	Switch agent
/model [name]	Switch model
/submodel [name]	Switch model for subagents only
/session [id]	Switch session
/sessions	List sessions
/new	New session
/compact	Toggle compact mode
/quit	Exit

Tasks

• Create src/cli/ink/commands/registry.ts
• Create command handlers
• Implement help command
• Implement agent/model/submodel switching with SelectMenu
• Implement session management commands
• Wire up command execution in InputLine

Acceptance Criteria

• All 10 commands work
• /help shows all commands
• /agent, /model, /submodel show selection menus
• Unknown commands show error

Estimated: 1 day

[randomm]

Completed - All 10 slash commands implemented and merged to dev