fix(tui): show subagent in agent picker when viewing subagent session

[aryasaatvik]

When viewing a subagent session (e.g., explore), the TUI agent picker now shows the subagent instead of "Agent not found".

Changes:

• Include subagents in agent picker only when viewing a subagent session
• Auto-select the subagent based on the session's first message
• Change agent storage from single global to per-session for better session isolation
• Remove redundant automatic model update effect