[FEATURE]: Add 'Context Files' expandable list to sidebar (TUI feature)

[AndrewZurn]

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Describe the enhancement you want to request

Hey there, super cool project, the TUI is my daily agent driver. Keep up the great work!

I love the new sidebar in the TUI as well; it got me to thinking about some other helpful information that could be displayed in it, and a potential 'fix' for the 'problem' I've described below.

Problem

I've found myself asking the agent a few times if it has read my AGENTS.md file (or other context files) into the context window, and almost always get back an answer of yes, or a list of the files it's read. But it's wasteful of tokens/chat space to have to ask for this confirmation, and could get lost in long chat sessions. Additionally, not knowing what files have all been read in a session could lead someone to become confused as to what the context window currently has in it and attempt to force it to read files more than once (again, wasting tokens).

Solution

Similar to displaying Modified Files in the sidebar, include a new section called Context Files that has an expandable list of files. This would include any system/global/project files the agent reads at initial load time, and then any files that are read during the session.

I'm a believer in "Show, Don't Tell", so I took a swing at what this could look like:

Disclaimer

I'm only a TUI user, and am not sure how this would fit into the other interfaces (I didn't see anything in the CONTRIBUTIONS file indicating what sort of thoughts need to go into a feature request to address the multiple interfaces OpenCode supports, and if a feature request would need to address all those or not), so if there is any guidance there, that'd be great.