[Feature] oclite session management and model selection commands

[randomm]

Summary

Add essential session and model management commands to oclite TUI.

Commands Needed

/sessions

• List previous sessions for selection
• Reuse existing opencode session storage backend
• Navigate with arrow keys, select with Enter

/new

• Start a new session with the current model
• Clear current conversation

/models

• List available LLM models
• Same list as opencode /models command
• Select to switch model for current session

/subagent-model

• New command (not in opencode)
• Same model list as /models
• Sets the SUBAGENT_MODEL for task delegation
• SUBAGENT_MODEL can also be set via env var

Session Behavior

• oclite with no args → new session with default model
• Default model = last used model (same mechanism as opencode)
• Backend storage already exists in opencode core

Related

• [Research] Lightweight TUI replacement - Raw ANSI vs React/Ink #27 - Lightweight TUI replacement
• [UI] Live Task Status Panel #19 - Live Task Status Panel

[randomm]

Completed in commit 39a5c91. Session management, model selection, and commands implemented.