[FEATURE]: Collapsible left rail with project & chat history navigation alongside tab support

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Problem

Currently, opencode has tabs (which are great), but there's no persistent left rail/sidebar for quick access to projects and previous chats. Users have to navigate away or lose easy access to their conversation history across sessions.

Proposed solution

Add a collapsible left rail that includes:

1. Projects – list of active/recent projects with ability to switch between them
2. Chat history – list of previous conversations, searchable and filterable by name or date
3. Ability to organize, rename, or archive chats

This would coexist alongside existing tab support, not replace it — tabs for working on multiple topics within a chat/project, left rail for switching between different projects/chats.

Why this matters

• Reduces context switching — navigate between conversations/projects without losing your place
• Keeps the UI intuitive — familiar pattern (similar to VS Code's sidebar activity bar)
• Complements tabs beautifully: tabs = "what I'm working on right now", left rail = "where am I doing it"

UX reference

Similar patterns used successfully by:

• VS Code (left panel for projects + chat history alongside tab system)
• Linear (project/navigator sidebar + top tabs)